Keep all Pokemon the same levels, including boxed ones. Trainers get additional Pokemon added to their team, also have significantly raised levels and difficulties. Wild Pokemon levels increased and evolved. (Only some of them evolved, not all of them.) Gym leaders have 6 pokemon with their previous ones evolved.
You already know what would happen with the Elite Four.
You can make some stuff happen with NG+'s lol. Personally, I think this feature would make things more harder and fun.
Perhaps their can be new wild Pokemon from older generations.
Yup, something like this. --- Xbox Live: KeroTenu PKMN FC: 5415-5334-2248