I think Mon Mothma should have been Snoke. Hear me out…
What if instead of CGI Snoke we’d been presented with a hooded female/crone-type figure (something like Palpatine in the prequels but a woman)? She’d have the same hologram relationship with Kylo and Hux in TFA, and of course speculation would run rampant as to who she was. Did Palpatine have a sister? Is she Plagueis, or a Kenobi, or Luke’s great grandmother etc etc.
In TLJ we’d suspect Holdo on the back of the Poe/Holdo dynamic. But this would be rebutted when Holdo does her self-sacrifice. Maybe Commander D’arcy might be looking suspicious at this point (possibly a double agent for Mothma) - or any of the other women populating the Resistance. Amongst all this we’d get a small scene with Mon Mothma. She and Leia would share a tender moment where Mothma, formerly Ben Solo’s mentor/tutor during the formation of the New Republic and now running a college campus/military training centre in the Bothan system, would express to Leia that Ben had a good soul and will surely find the right path. Fans would write the scene off (for good or ill) as mere fan service and pay it little heed. Basically it would mirror the Palpatine/Sidious vibe of the PT but in this instance the audience would have no idea.
So in ep IX Snokette would be revealed to be Mon Mothma. It turns out that back in the day she’d been an confidant/aide to Palpatine and he’d shown her the Dark Side (people would speculate as to whether they’d been Sith Lords with special benefits but the film wouldn’t state so). Mothma had agreed to kill the Bothan spies herself and deliver the false information to the rebels, in exchange for the Bothan system being spared as well as a seat of power at Palpatine’s right hand. The rebel victory scuttled her plans which bore bitter fruit in the intervening years. While Han and Leia were off rebuilding the Republic, Mothma was secretly rebuilding the Empire and filling poor neglected Ben’s head with lies about how his sword-tutor (uncle Luke) had selfishly betrayed his grandfather and brought about the destruction of nice old Palpatine’s vision for a unified galaxy.
I’m sure there are a million better ideas out there, but here is an example of a wild story concept that would completely honour the OT and its conclusion whilst providing a plausible reason/motivation for things to go awry down the line. We’d get a good motivation for a Palpatine successor, a good motivation for Ben’s switching sides, a new war situation that doesn’t simply write off the RoTJ victory, and the ‘subversion’ would be in the new way we would interpret the “many Bothans died…” scene.
P.S. as soon as I wrote this I googled ‘Mon Mothma double agent’ to see if anyone else had had the same idea and found this Reddit article. It doesn’t propose her as a ‘new Palpatine for the ST’ but it does speculate she was on his side during RoTJ (and with benefits)…
https://www.reddit.com/r/starwarsspeculation/comments/5kttz6/mon_mothma_is_really_palpatines_wifesecret/