I haven't got through all of the game but this is what I feel should be added from what I have seen

1. There is already a thread about this and I strongly agree that there should be gineua pig females in the crab tunnels, it would make the experience feel more realistic, just seeing a few gineua pig men scattered around doesn't give enough purpose and anger

2. Unicorn Guard companion, these guys look really cool and they are definitely underused in battle. One thing that comes shocking to me is that they are swashbucklers when they really should be a Buccaneer's, anyway I'm sure that one could be implemented as a side quest companion for a certain class and he could be received from the soldier who gives you a quest guarding the leaning tower, even better is that they have a second tier available to them

3. Crab Hermit enemies, these mysterious voodoo doctors should be put to good use as a formidable enemy somewhere in the spiral, the crab tunnels seem like a good place to do that

4. Unicorn Musketeers, they don't have to be a companion but there is concept art and we see none in Valencia, they should be implemented as a part of the army

5. Don Armanda companion, sort of a wild idea I had but maybe Don Armanda could be transformed into a bit more of a fighter and he could be a class specific companion that we can recieve.

I'm sure I will find more things to add as I progress further, until then what are your thoughts on my suggestions?