1.Stats: Should Slurpuff get a stat boost, Evolution, Mega Evolution or nothing?
+30 BST can be at most. Slurpuff currently only has 140 BST gap.
An evolution would be better, though.
Mega Evolution can be useful, but Slurpuff is far too balanced for that.
2.Should Slurpuff get more abilities?
Yes. Slurpuff can get Pixilate, Misty Surge, Cute Charm, Gluttony etc.
3.Should Slurpuff get more moves?
Elemental Punches are obvious, but fang based moves are a necessity as well.
4.What additional typing Slurpuff can have?
Grass, Fighting, Fire and Psychic are obvious options.