there has to be a good muckmorpher deck

Muckmorpher let’s you cheat out big minion effects early. That idea sounds like it has a lot of potential. This deck uses it to try and play 5+ Zilliax, and possibly even Malygos someone down with burst damage.

Gameplan

You are a control deck that can theoretically Eureka! out a Malygos and play x2 Totemic Smash + x2 Lightning Bolt for lethal if need be. You’re rarely the aggressor. As a result, this build probably just loses to Control Warrior because they armor past your 30 damage burst and there’s not much you can do about it. On the other hand, you have 2 board clears in Lightning Storm and Hagatha's Scheme in a meta that is very light on board clears, and you play ~5 Zilliax, so you have a lot of sustain.

Card choices

As this is a new archetype, let’s break down the core packages:

Muckmorpher package:

• Zilliax – Zilliax number 1. hitting this off Muckmorpher is a straight up improvement.
• x2 Muckmorpher – duh. Zilliax number 2 and 3.
• Al'Akir the Windlord – also probably just better than the original when you Muckmorpher it, and obviously insane tempo.
• x2 Walking Fountain – no divine shield, so Muckmorpher‘s 4 health is weaker here, but still very strong. Zilliax number 4 and 5.
• x2 Eureka! – whatever you’re trying to cheat out, you’re going to want something you can play when you draw it and it can’t be Muckmorphered.

It’s unclear how powerful Ancestral Spirit is with Muckmorpher/Eureka! at this point. It seems possible that it is also core.

I don’t consider Malygos core. I like the idea of running it to have a burst win condition, but I don’t think it defines Muckmorpher decks.

Control Package:

• x2 Lightning Storm – any Muckmorpher deck is gonna cut early game minions, so this is important to keep your opponent’s early game in check
• x2 Hex – best single target removal in the game
• x2 Hagatha's Scheme – probably the best AoE in the game right now.
• x2 Rain of Toads – arguably not core, but as any Muckmorpher deck is going to be minion-light, throwing a bunch of taunt stats on the board is valuable defensively and offensively.

Flex Slots:

• Zap! – early game removal that synergizes with Likkim. Also makes you feel better about hitting Malygos with Muckmorpher on curve.
• Lightning Bolt/Totemic Smash – reasonable early board control, and represents a win condition with Malygos.
• Ancestral Spirit – cheating out big dudes and making them revive themselves seems powerful. On 7 if you play this on a Muckmorpher that hit one of your windfury minions you can basically attack with it 4 times.
• Likkim – reasonable early board control/extra burst damage. 
• Far Sight – a lot of these flex slots are just sort of filler, and what could be better filler than draw that gets you to your next card.
• Haunting Visions – maybe you’ll find a third Lightning Bolt for lethal MonkaS
• Malygos – a natural thought in a deck that’s built around cheating minions out.

Other options:

• Big Bad Voodoo – similar to Ancestral Spirit. If you Muckmorpher into a windfury guy on 7, you can theoretically kill 2 guys, and summon a 9 drop. Sounds pretty powerful. I like Ancestral Spirit a lot better because I get more of the good cards I put in my deck on purpose, but this might be better than some of the filler slots right now.
• Witch's Brew – I think we have enough healing with all our Zilliaxs, but it’s an option.
• Hagatha the Witch – this isn’t in my list because we’re only playing 7 minions and they’re kind of big, so there’s not a lot of synergy. But maybe it’s just insane not to play a hero card in a value deck when you have access to one.
• Big Bad Archmage – seems like a meme, but it’s a spicy one. 4/4 and a 6 drop on 5 with the threat of making more 6 drops is pretty good if your opponent doesn’t just kill you.
• Ysera – lots of value represents another genuine win condition now that the infinite value death knight days are behind us. Definitely one to consider.

If you want to a non-Malygos version you could try something like: