Following the tradition of other mobile games, what do you think of the idea of different types of food (coloured differently) for each element? So you can only feed 'purple berries' to Magic monsters, and 'green berries' to nature monsters, etc..
I also like the idea of needing to feed your monsters regularly to maintain their accuracy in battle - monsters' accuracy decreses over time unless they are fed correctly... (so accuracy will not be random anymore).