1.Stats: Should Muk get a stat boost, Evolution, Mega Evolution or nothing?
Muk can get at most +40 BST.
An evo that is similar to Rhyperior can work.
A Mega would be fine.
2.Should Muk get more abilities?
Limber is an option.
3.Should Muk get more moves?
Recover, almost certainly. Drain Punch is another option.
4.What additional typing Muk can have?
Ground, Fire, Electric, Ghost and Water options. Water is largely thematic.