Inspired by a certain post - I'll attach the link soon.
In the anime, we often saw pokemons evolving half-way through a battle, and turn the tables in many ways.
In the games, however, no matter how many levels we gain, we always have to evolve AFTER the battle is over.
Do you want pokemons to evolve during battle? Like, they could evolve the moment they gain the right level, or you could use a stone on them when you think you need it.
For me, I want pokemons to evolve during a battle. Then it would be totally unpredictable - and much more exciting :)