Great mod, I like the texture and would prefer for the cows to not spawn naturally as I too like the challenge As for feedback I'd say drop the ability to get potions from cows (although awkward is good and makes sense, so keep that:) and think about adding other animals such as chickens/ non-zombie pigmen etc. Also maybe instead of the egg being a random drop make egg shells drop and have to craft the egg, maybe out of eight shells and a leather in the centre. This would allow you to add different animals by putting a different item in the centre of the crafting grid
1. For now I'll drop the potion idea but in the next update I'll be fixing the bugs for when the nether cow is walking over a soul sand field.
2. I'll probably be adding two new cows in the future one would be the lava cow as suggested and I am also thinking about adding an enchanted cow(Not going to spoil)
3. I like your idea of shells so I'll consider making them okay.
Here is a list of cows and the word next to them describes their nature.
Nether Cow - Passive
Lava Cow - Passive(If hit sets player on fire)
Ender Cow - Passive(If hit it teleports randomly)
XP Cow - Passive(A nice asset next to an enchantment table) Plus there is an easter egg for people.
Ghost Cow - Neutral(If looked at will come at player till one or the other dies)
Zambie Cow - Hostile(Not a zombie but a zambie it attacks other cows)