I apologise in advance for the walls of text. I'm in assessment mode right now...
For me there's five that come to mind. Three that I'm desperate to see recontinued, so that I can see my plans realised, and two that while I still see potential in, I have my reasons for not being as keen to revisit.
The two are Powder Keg and Venice. Both are concepts I loved, both in my personal character and the actual RP itself. With Powder Keg, I had the idea of RPing as the surviving little girl my initial Gunpowder character set out to save. I never got around to finishing the Gunpowder story though, and revisiting that character is the part that puts me off. As for Venice, the character I created is very reliant on the cast she's surrounded by, so that would really affect how fun she'd be to play.
As for the three; I've got 'Cracks: a sinnoh Journey', 'As the Dust Settles' and 'Titans Reborn'. All three were some of the major inspirations for the creation of this thread in the first place.
For Titans, I was exploring the idea of this character who's had everything go her way in life; growing up in suburbia, having a natural talent, perfect romantic partner, nuclear family and how becoming a superhero took that away from her. With a 5 year time skip, Titans: Reborn let me explore what happens after all that damage has happened, and the superhero life comes calling again after ruining her life. It made for a much more interesting character, especially with the cast she had around her.
As the Dust Settles is a good one, since I had 4 characters to juggle. Particularly with my all action maverick character, Lars, who fought almost exclusively using his legs. The plan was for him to learn to work with his team, eventually sacrificing his legs in order to save his teammate, and ending up in a wheelchair for the latter third of the RP. He was a much simpler character, so he was fun to write, without having to juggle complex motivations. Time was the main thing that set me back on finishing that one, now the RP is finished and my characters arc literally ended in a cliff hanger.
Cracks is one I can still realistically see being redone. Looking at you Gimmepie. It's not a sequel RP like most of the others, but it's one I put a lot of planning into. My orphan character, Logan, was going to catch a bunch of pokemon, each teaching him a different aspect of family life. Sacrifice, forgiveness, unconditional love etc. The first one died as the unfortunate result of poor communication, which is even more detrimental in an RP with four PCs all travelling together.