Pam/Winnie had more personality than the top 4 together. Only Pwinnie & Youtubers seemed to have personalities (Sorry other racers)
IMO, Max and Katie were pretty much up there in terms of personality. At least for Max, he embraced the "villain" role, somewhat similar to Colin of TAR5 fame. Jennifer and Caroline might get an honorable mention, but other than that, meh. J/C did have a lot more personality compared to 65% of the racers since 2001.
I think if anyone would return outside the YouTube team, Pam/Winnie, and whomever else, it would be John & Jessica. Boring, I guess so, but based on their run and their decision that ended their run, it would make a lot of sense.
As for a racers return season, 25 would be great, but the show didn't really do much for "landmark" seasons 5, 10, 15, and 20. Although TAR5 didn't need anything; it was historic on its own. TAR10 was great due to the most diverse cast ever, IMO.
I think they might target 26 before 25, due to a spring run (and May sweeps). But in all likelihood, I would not be surprised to see new racers over the next five seasons.