You can recruit legendaries and do more things.
you first have to get to midnight forest after beating darkrai. Bring lots of food. Rotom should appear around B15. :P
You can't actually beat the game, but the farthest you can go is defeating Darkrai. You can be considered beating it if you beat him.
Finish the exploration of Aegis Cave with Team Charm, then after beating Regigigas, it will be unlocked.
Go to luminous cave with some other Pokemon, if their level is high enough you can evolve. for your starter Pokemon you wont be able to evolve until you have finished the entire game/ Beating darkrai.
By Beating Mew in the Mystery Jungle. To go to the Mystery Jungle, it is a job. I forgot how to get it though.
You have to beat Dialga then go through a series of events that lead to beating Darkrai. After graduating from the guild and kicking Darkrai to another area of time( there is a way to get Darkrai, get secret slab or mystery part then go to miracle sea third floor with either the secret slab or mystery part and Darkrai will be there)you can evolve.
Do alot of missions and I am on diamond rank, having trouble beating dusknoir (the part before primal dialga).
you can get any Pokemon there when you have beaten darkrai: electivire charizard gliscor etc
by beating chatot
There's really nothing after that sadly
Porygon-Z, which you can capture in the oil field hideout after beating darkrai.
Porygon-Z. It is in the Oil Field Hideout after beating Darkrai.