Dany going mad late in the game has always been a leading theory. If that's her path, I'm surprised the show has taken this long to do it. There have been hints all along, but she's held together reasonably well. A main character doing a Jekyll and Hyde at the last minute is awfully gimmicky, but this is probably driven by the artificial time constraints under which the show now labors. And again, for the umpteenth time, the subtlety and complexity of the plot has continued to decline since the show outran the books.
The show hasn't signaled that Jon is unstable, but if I were writing it, I'd throw an ominous sign or two into the mix in the last episode. I wouldn't have Jon commit any grave crimes, but I'd do just enough to give people an unsettling feeling as the show wrapped up.
Dany has recently lost:
-2 “children” (dragons)
-Ser Jorah
-best friend Missandei
-Most of the Dothraki horde
They helped keep her grounded.
How much of a threat is Dany without even a single healthy dragon?