As we know trainer towers have brought out a huge amount of levelling benefit for the players and some have even devised a way to train efficiently, and one way is by lending the Pokémon of said level to help level your other mons .
But since trainer tower in some region are somewhat far from a poke center, the back and forth consumes more time while you could spend that time levelling instead.
So for convenience I suggest that trading should be available inside the trainers tower.