Tyrells plan if Robert didnt die

Hi, i was wondering for a while, if things didnt go south quickly, meaning if robert didnt die, if people didnt find out Jamie and Cersei and so on, what was Tyrell's plan overall? Joffrey was lock with Starks, Tommen is useless or maybe semi useful but not as much would Sansa.

Margery always wanted to be queen as i can understand. IF that wouldnt be option would tyrells just marry tommen and margery and gain influence in small council and try their luck in next generation?

I dont remember they looking for targerian siblings so that cannot be their plan, so what?