The upcoming Taverns of Time event will also greatly affect Arena as it will also bring a decent chunk of exclusive cards to the format! Many of them have extremely high variance and effects that are not obvious at first, so it’s worth familiarizing yourself with them before they go live. While their eventual impact on the metagame will mostly depend on which bucket they will be assigned to, having a general idea about their strengths and weaknesses can be very useful when preparing to get those elusive Lightforge Keys.
Neutral Cards
Temporal Anomaly – A solid, though slightly class-dependent card that is very similar to Druid’s Tortollan Forager. It’s worth noting that you lose its ability if you have in in your opening hand. It will greatly depend upon what other cards it will go up against.
Murozond – A really interesting and awesome card, and one that is definitely not recommended if you’re playing on mobile. You will overdraw a lot, will have too many options to consider, but the effect is obviously insane on the board. If you can keep up, it is top-tier premium level, and if you can’t, you better practice, because this will be an auto-pick for sure. Plus points for the lovely little anagram on Nozdormu. 
Possibility Seeker – It is essentially a Chillwind Yeti with added variance, and you’ll rarely be able to engineer a shuffle on your terms. The effect isn’t a downside per se, but it will bail you out just as often as it will force you to put that Spikeridged Steed back in just before you could play it. Notably, you can upgrade the Coin with it.
Timeline Witness – A useful effect, but the minion has to survive to make it happen – it very much feels like a win-more card in tempo decks to me. It will be one of the better 3 mana 2/4 cards in the pool, but that isn’t really saying much.
Wildlands Adventurer – A card with obscenely high variance that will certainly ruin a few of your runs. It is vanilla-statted and replaces itself, making it a strong choice regardless of what you’ll get out of it.
Stasis Dragon – It’s not as bad as it looks. You pass your sixth turn in order to get a guaranteed value-trade on the eighth, at least if you play it on curve. However, being able to afford such a tempo deficit is not guaranteed. Mages might be able to make it with their plethora of expensive board clears, but it’s definitely not a safe pick. It’s the Paladin’s Steed turn which you would pass, which is never a safe decision to make – and it will most likely make you float even more mana if you play it later, which is a concerning possibility. Exercise caution.
Chromie – You only sacrifice one stat for an amazing potential upside, though it is quite possible that you never draw the Epoch over the course of the game. I would have expected it to be a Legendary, being a named character, but none of the Arena-exclusive cards have such a gem. Fal'dorei Strider is a pretty decent Rogue card, this should also be an alright option. 
Infinite Murloc – A much cuter version of Jade Idol – it’s still not good though as the payoff is impossible to guarantee.
Timeway Wanderer – A really interesting card that immediately reminded me of Ethereal Conjurer, which has historically been fantastic in Arena. If you directly compare the two, this one grants you five mana at the cost of your next draw – both this and the statline make me think that it’s really only worth it in a tempo-oriented deck. Certainly worth experimenting with!
Timebound Giant – It’s only going to cost seven mana by turn 8 and it only gets better from there, especially in the classes that have great drawing options.
Druid Cards
Flash Forward – Symmetrical effects like this are generally very bad as your opponent gets to take advantage of them first.
Harbinger of Catastrophe – A similar story, though the effect triggers on the start of your turn. The minion doesn’t lose any stats for the ability, so it is costed as a net-neutral one, but it is unplayable when you are behind and still potentially devastating when you’re ahead. It’s a win-more card that might lose the game: yuck.
Hunter Cards
Chronoshot – One extra mana for a Sap that takes away two from your opponent in a class that is going for tempo plays all the time, you say? This card is close to broken, especially when you consider how it can literally make the most expensive minions unplayable.
Infinite Wolf – It’s a really interesting and pretty good card: it functions as a pseudo-Kill Command the first time around and only gets better from there if you treat it as a removal spell. Eventually, it will simply be a huge threat that you won’t attack with so that you can send it towards the dome the following turn. The effect is certainly worth the price.
Mage Cards
Consider the Past – Devour Mind and Cabalist's Tome were great cards in the recent arena metas, so this will probably be quite good as well: it’s certainly undercosted for what it does, which is always very promising for a value card.
Cavern Dreamer – Absolutely disgusting. At worst, it’s a Primordial Glyph for all classes that you roped on, and it gains incredible value if it can survive one more turn. It’s also cheap enough that you can toss it down after a board clear. A must-pick in almost all situations.
Paladin Cards
Blessing of Aeons – This is Paladin’s take on Feral Gibberer and it can certainly be just as game-winning under the right circumstances. It’s extremely cheap and flexible, and can turn a small token into a massive threat at a very minor cost. It’s useless if you’re behind, but if it is too conservatively bucketed, it could definitely be a nifty little tool to pick up during your drafts.
Bronze Broodmother – The extra token is nice to have, but it certainly is no 5 mana 6/6, just like how Stand Against Darkness never compared favorably to Stranglethorn Tiger in Arena. It’s barely above average.
Priest Cards
Reminisce – A conditional 2 mana draw 2: Devour Mind is excellent in Arena, so this is certainly at least average, though the possibility of grabbing the Coin certainly weighs it down.
Ripple in Time – Great flexibility, though the sheer size of the minion pool means that you will often end up with something mediocre. Otherwise, it can get you something insane (like Northshire Cleric or Shadow Ascendant) or a big fatty with an extra mana added to its cost. There may be way too many mediocre outcomes to make this the right choice in your draft.
Rogue Cards
Deja Vu – Absolutely deck-dependent, but it is essentially a more consistent Hallucination, which can also be an OK choice under the right circumstances.
Thief of Futures – An excellent card that replaces itself and provides you information about your opponent’s current options. Self-replacing cards like Stonehill Defender and Bone Drake have been overperforming currently in the Arena, and this has some extra added benefit to it – in a tempo class that will have an easier time making up for its missing stats.
Shaman Cards
Master of Realities – Giving up a single Attack point from Boulderfist Ogre nets you an incredible effect (that also applies to totems and tokens). Fantastic and premium card, though ever so slightly hindered by the fact that so many of Shaman’s top-tier options are geared towards a control strategy. Still, it is an unbelievable effect.
Stasis Elemental – Again, for a control-oriented Shaman deck, this could provide some valuable stall until you piece together your desired AoE setup. It reminds me of Fel Reaver, which has only seen fringe play in a few oddball control decks back in the day – and it also makes Shaman’s useless Freeze-related cards better, not that this should make you more inclined to draft those if they come up. It would have made a very interesting option in Constructed.
Warlock Cards
Rift Warden – How would you like a two-card seven-mana Cairne Bloodhoof? Especially in a class that doesn’t really draft large minions currently. It’s small enough to just be ignored when played on curve and would deny you the discarded minion’s Battlecry. You do get to control the effect to some extent, but it seems like a volatile and underwhelming draft option. There’s a reason why Coffin Crasher also fell flat on its face.
Grasp the Future – This is a weird one with very polarized initial reactions. Is this more of a Lunar Visions or a Primordial Glyph? It’s really hard to judge without seeing it in action. My main concern is that it’s in a class with natural card draw attached to it already, and one that heavily relies on using its life as a resource, so the prospect of sacrificing tempo and therefore my life for card draw doesn’t seem particularly appealing. Of course, the later you play it, the less this applies and therefore the better the card gets: I’d certainly not pick it over a Despicable Dreadlord, let’s just say that.
Warrior Cards
Fatecleaver – The text will rarely come into play, but a 4 mana 4/2 weapon is just good on its own for Warrior.
Draconic Herald – It really depends on the Discover choices, but anything with Rush, Charge or Taunt can be a game-changer once you apply this buff. Warrior is in a bit of a slump again though, and there’s so much shake-up going on that it’s impossible to tell whether this will be enough of a help to make it viable again.
The regular card that suddenly became great with this arena event? Dragonslayer. It’s a killjoy, I mean, tempo swing that has worked wonders for me so far.
not sure you read cavern dreamer correctly. it doesn’t get you a spell that now costs 2 less, it gets you a spell that originally costs 2 or less. That card pool is pretty good though. I could also be misinterpreting what you meant there with the primordial glyph comparison.
“Chromie: I would have expected it to be a Legendary, being a named character, but none of the Arena-exclusive cards have such a gem. ”
I suspect none of the cards have gems because they are uncollectible as I seem to remember Blizzard have been removing gems from uncollectible cards that used to have them. Chromie still has the shape of a legendary card, though, with the dragon border, so i suspect will still be treated as one.
Any information on how often those cards will be offered or if they will have any appearance bonuses?
If only!
Chronoshot IS broken, ESPECIALLY in Arena. Sap from Rogue wins games on its own. Chronoshot powercreeps it, and can even make 9 and 10 drops a dead card in the opponent’s hand. There’s a reason why Hunter’s secrets are secrets, if they could be targetable spells for 1 extra mana, they would be broken. And this is essentially Freezing Trap for one extra mana to make it targetable, which is huge.