So R* confirmed a while back that purchasable property won't be on the cards. However, they did say that you would be able to spend your hard-earned money on toys of sorts.
Perhaps purchasable furniture for your properties? The ability to change rooms perhaps? It would be pretty sweet to give Michael a nice arcade room in his pad. Or perhaps an upgraded trailer for Trevor?
I'd imagine that cars will play a role in some way, perhaps some small scale customisation to your cars? Being able to purchase cars for your garage?
More likely though, I think that R* will be focusing on weapons, weapon modifications, weapon ammo and such.
What do you recon?