Prestige system on OSRS because its not been out long and there would be less pain with resetting so early on. Also good for Pkers and skillers who just play for that purpose.
Possible updated interface such as the one currently on Rs3 but a much much easier version so you can just mess around and choose your own OSRS custom layout.
Summoning just for pets, not as a skill but a revamped summoning interface for new pets that are possibly coming into the game. e.g. the godwars pets if they are released.
Joint highscores. So for example if you have 200million exp in prayer on RS3 and you have 100million on OSRS it would be on this joint highscores as you having 300million exp.
Bonds just for membership on OSRS but you dont buy them from people who have bought for real money, these can literally be bought from a shop for Rs gold.
Of course you will have to solve the botting issue and add free to play worlds in the future