I think Finn was probably envisioned to be a positive romantic lead with *someone* in both TFA and TLJ, and then had that lost when TROS was made. I tend to think that Abrams was envisioning him as Rey’s love interest in a sweet kind of romance rather than a tension filled one, while Johnson was clearly trying to make a Rose his love interest (I think largely to kill of any chance at Rey and Finn as a romantic option. Isaac had fun teasing the idea of Finn and Poe, but I think neither Johnson nor Abrams were interested in that route, considering Rose and Zorri both exist, while the TFA elements between them seem to have been the result of improv and chemistry more than anything else.
Of course, basically *any* other romance would have been better for the Sequel Trilogy than the Reylo we wound up getting, and I’d still rank FinnPoe ahead of FinnRose in terms of possibilities strictly on the basis that Johnson made the latter the most tepid thing I’ve ever seen, somehow. I was personally more interested in Finn and Rey though... and that probably is a sign of my pro-Finn bias, since being the love interest for the main Force character feels to me like the kind of thing that couldn’t have been swept under the rug by Kylo-obsessed creatives.
People wonder if Jannah was meant to be another romance interest fro Finn, but I don’t think that’s in the final cut, at least. She may have been conceived for the role, and she definitely feels like an Abrams approved counterpart to Rose, but I wouldn’t be surprised if Abrams moved away from that possibility after realizing that it was getting a bit ridiculous.
As to the Stormtrooper rebellion idea, I had high hopes something like that would happen, and something vaguely like it is in the a Duel of the Fates script, but I don’t think LFL ever really considered the idea seriously themselves. I mean...
...This scene was the only part of TLJ’s production that ever seemed even vaguely aware of the drama posed by the Stormtroopers... and “vaguely” is the right word for it, I think, since the delete scene is really more interested in Phasma betraying the First Order, rather than in Finn pointing out any of the horror of the Stormtrooper program. And since Johnson’s other deleted scene idea dealing with Stormtroopers was a comedy moment where Tom Hardy slaps John Boyega's ass, and since he felt that Rose should lecture Finn (the escaped child slave soldier) on the evils of slavery, it feels like he never really cottoned onto the actual nature of Finn’s background. He was probably too busy feeling bad for the school shooter with the nice hair he gave a shirtless scene to.
Trevorrow’s script includes “Brutetroopers” who help give Finn leverage to start *some* kind of rebellion involving the Stormtroopers, but even it seems to be shying away from just making him the focal point and fuse for that story... and then Jannah and her barely seen crew are all that remains in TROS’s script.
I’ll confess I sometimes think there may have been a small amount of malice towards Finn on LFL’s part, not out of anything spiteful, but maybe out of a believe that Finn was some weird, superfluous creation by Abrams distracting from who they thought the male lead should be - Kylo. Kennedy pushes for Driver early on, and both Kylo and Finn emerged from the original male lead, with Kylo keeping that character heritage as a Solo. Abrams and Kasdan supposedly knocked out TFA’s script after months of issues when they kicked out everyone else and just worked it out themselves. I think it’s possible that LFL handed over a script to Abrams and Kasdan while already planning for Driver as a Solo kid male lead, then simply refused to change their viewpoint when Abrams made a movie with a different (more sympathetic and compelling) male lead.