I have finished all heists as leader, it said afterwards that lester will call me with details about a new heists, but I havent got the call yet after a long time, what does he say in this next call? And what is there left to do after this point in gta?
Heists are but one of the many things that can be done on GTA Online. In the future (if) when more heists are added he'll give you a call and give you the details as usual. The basis for this is that the call you receive from Lester letting you know he'll be in touch is the same generic call he gives after every heist.
As far as what there is to do still, I would say that it's a lot like playing real life, there is no "end game" in GTA besides owning a nice place to live and buying stuff you like, so just have fun!