Who would become King if Tommen were to be killed, which is something I see happening sooner rather than later? Here's an interesting article that goes into that discussion. What does everyone think?
https://winteriscoming.net/2016/05/13/game-of-thrones-theorycrafting-who-becomes-king-if-tommen-dies/?utm_campaign=WiC+Daily&utm_source=FanSided+Daily&utm_medium=email
This is something that I guess fans of the show thought about, but characters of the show didn't. Even though someone at court should bring up the question and provide also an answer, Tommen is young and all but so was Myrcella and she died, so the succession must be secured.
Robert Baratheon started his own dinasty (well, that lasted long), I don't know if you can go back to his ancestors since they never were kings to begin with. You can do this trick in a scenario where the Mad King dies but the rebellion is defeated so you can go "Ok, Targaryens are ruling since forever, lets' go back in time to find out some other heir, cousin or whatever", but you can't retro-actively consider the Barathon family a ruling one.
It's really a puzzle that has no answer, if at all, it opens a power vacum where everyone would claim the throne. The Tyrrels would probably suggest Margaery taking over until she remarries for example, but she's in prison.
I'd suggest that Littlefinger has a secret family tree certificate hidden somewhere to claim the throne by rights, if only he didn't have blatantly anything to do with Myrcella's death.
Pardon me if I missed any speculation on this but since the Lannister's are obviously trying to plot something against the High Sparrow do you think he would allow himself to be killed as a distraction while his minions take out Tommen simultaneously?
I don't think that the end game of the High Sparrow is to kill Tommen, I think he would just be content to control him (as he's shown to try and basically manage to do in the last episode), so that in return the king would sanction everything the Faith Militants do. If push comes to shove they're ready to get violent, but I think their aim is just to re-estabilish their ways, not to politically rule.