Maybe not the Reach and the Riverlands. The Tyrells only had power because Aegon gave it to him and the Riverlands are doing better under Aegon. I do find it charming that for a series thats grittier and darker conquered people are so damn passive.
Now if it had come out that the Maesters killed the dragons as revenge for the conquest that would've been cool.
The Tyrells would, in such a circumstance, be facing Rebellions up the ass from houses whose male heirs can directly trace their line to Garth the Greenhand, as opposed to just the female line like House Tyrell.
The Riverlands were infamously fractious and had some history of backstabbing whoever liberated them from the latest non-Riverlander oppressor, so I could see even them rebelling against vulnerable Targs (as they eventually ultimately did, to fatal results for the latter, in Robert's Rebellion). Harren the Black was only the third Iron King to rule the Riverlands, his grandpa
Harwyn 'Hardhand' being the first; somehow despite the Ironborn having been well established as the sea-niggers of the setting by that point, many Riverlords sided with Hardhand when he fought to conquer their homes from the Storm King (who they at least mostly shared a religion with). The arch-traitor that time was
Lothar Bracken, who tried to rebel & set himself up as an independent river king after helping the Hoares buckbreak the Durrandons & his own neighbor the Blackwoods, and was in turn duly curbstomped by Hardhand & starved to death in a cage.
(This is also Case #1,000,000 in the long history of Brackens obviously being the house Martin hates most in the setting & the Blackwoods being the
real saintly Mary Sues he favors BTW, you can tell who he intends to be the good & bad guys in any given conflict involving the Riverlands based on who each of these houses side with.)
The last time the Riverlands were canonically unified & independent under one of their own was
under the Teagues, who faced constant rebellions (because their founder was an upjumped sellsword who forcibly united the region with a huge mercenary army, an even worse background than the Tyrells) and were eventually destroyed
when the Durrandons backed the Blackwoods in a rebellion against them despite both the Teagues & Durrandons being Seveners while the Blackwoods are famously Old Gods followers (something which had no precedent from the RL Middle Ages to speak of really, even the famous Polish-Lithuanian alliance to topple the Teutonic Order only got going
after the Lithuanians stopped being pagans).
The last time the Riverlands were canonically unified, independent &
stable under one of their own was in the time of
House Justman, which is another case of Martin not allowing good to succeed (or at best, not for long before coming to a ridiculously brutal & senseless end) in his work no matter how little sense that might make.
The Justmans' founder Benedict was a Bracken-Blackwood bastard, but such a cool giganigga that he miraculously managed to get the two to set aside their millennia-old grudge & become his first supporters on the long road to unify the Riverlands for the first time since the Andals destroyed House Mudd, which took him 30 years. By reputation the Justmans tempered their Ned or Stannis-like sense of justice (hence their name) with Arryn-esque honor & the compassion of the Gardeners to rule successfully, they're noted as having not only kept the Brackens/Blackwoods at peace but also expanded their kingdom as far as the Neck in the north & the Blackwater's mouth (where KL would later be built).
So naturally, such a benevolent house had to be exterminated by the Ironborn of all people (who first somehow buckbroke the Justmans to the point of forcing them to pay tribute despite the Riverlands being a juggernaut compared to the Iron Isles,
then treacherously butchered the last Justman king's underage sons and killed him when he tried to avenge them) because nobody can be allowed to have nice things in the grimdarkness of Westeros, at least not for long. The Justmans ruled for 300 years, which might sound a lot until you recall that in this setting, the fucking Freys are still considered a young & upjumped house at 'only' 600 years old. Or IOW, this house of Riverlander Neds/Jon Snows basically only got to exist for a blink of an eye in Westerosi history.
Plot-Letters and he called it off. It's not known what was in the letter but in-universe it's speculated that it was some crazy dornish shit like they had Rhaenys alive but tortured and they would only end her suffering if they made the lizards go away or sending all of Dorne's gold to buy a faceless man to kill Aegon's heir Aenys.
