Just some thoughts i had this early morning:
Will any monsters have magic abilities?
Self-healing / regenerating monsters ?
Can monsters breed ?
Can any monsters be equipped with armor, weapons and stuff you buy ??
Will monsters change appearance when higher levels are reached ??
If monsters can fight as a group, some defensive/shield monsters would be nice!
If a monster gets ill, can you care for it and cure it with bought [ spam word ] ??
Will some monsters be picky about what they eat ??
Can monsters have pets or toys ?? or any other recreational qualities that can
help a monsters mental state ??
edit: If you don't feed your monsters, will they eat eachother ??