Tommen IS going to die. Maybe not this episode, but soon. And Cersei can't die until he does so lets get these dominoes falling. Please. As for the inheritance:
1) Gendry/ Edric Storm inherits with the backing of the Storm Lands. We don't really have a named character championing the Baratheon line since Davos is clearly under the Stark banner for now. Although maybe Jon refuses his call for justice in regards to Melisandre and he heads south to Storm's End. Low Probability.
2) The Lannisters argue that Tommen's nearest male relative should inherit and Kevan becomes King. This would be relatively easy to do as he is the overall Lannister commander and Hand of the King. The #2 guy has a good shot. If the High Sparrow endorses him (Assuming he is alive) it would be a shoe in. Moderate odds.
3) The Tyrells argue that Margery is Queen and should continue to rule. I don't think this is standard practice in Westeros. I can't think of another time this happened in the past but with the situation as it is, and if the Lannisters are dead (Cersei -clearly Margery's plan- Tommen, maybe Kevan?) or returned to Casterly Rock, continuity is better than all out civil war.
4) Tommen and Cersei die and Dany lands in Dorne, proclaiming herself Queen. The Tyrells do what they normally do and try to marry Margary to Dany but then realize that won't work and just support Dany in order to avoid war with Dorne.
5) Littelfinger marries Sansa, becomes Warden of the North and names himself King.