I think an Trystane who isn't Aegon but who does pretty much everything Aegon does fits a little bit better, just because it's a bit less backstory to explain. It would just be that Doran was a co-conspirator with Varys and Illyrio (I think it was intentional that Varys mentioned multiple colleagues last episode) and once they got word that Viserys and Drogo were dead, the plan shifted away from them and towards Dany, who they planned to tie down with a marriage to Trystane. When Trystane finds out he's going to marry Dany when she arrives, he decides he should start conquering now to impress her when she gets here, and attacks Storm's End, just like Aegon. He basically becomes a weird fusion of Aegon, Quentyn, and Arianne all in one.
Regarding Aegon, Euron/Victarion, Lady Stoneheart, and any of the Oldtown business -
The best way to understand these characters and subplots (and how they will or won't be handled in the adaptation) is in terms of 'jobs to be done.' And since we've only got the beginning for all of those arcs, that's REALLY difficult for anyone in a bookreader's position to guess at right now. For example, it's possible that the entire purpose of Aegon is to provide a possible marriage candidate for the Three Queens to fight over while also offing (?) Loras. It's possible that the entire purpose for Lady Stoneheart is to kill Jaime. But none of these things have happened yet, and it's totally possible for something
wildly different to perform those same functions within the story without really resembling the existing thing at all (e.g. whatever 'jobs' Mance Rayder was going to perform for the story will now likely be done by someone else).
I don't think it's necessarily correct to assume that any character's going to come along who is a close analogue to fAegon, basically, or it's possible that fAegon is just gonna be held for Season 6, or they've still kept him a secret somehow. Who knows.