7) The Original Harrison Wells
While the Earth-2 Harrison Wells returning is absolutely essential, Barry stopping Reverse-Flash in the past should mean that the villain never murders the original Wells and takes his place. As a result, he should still be alive and well in the new timeline, and The Flash realizing he has to essentially kill him all over again by putting things back to how they were would be something which really tests the hero.
That aside though, and it would also be a lot of fun to spend some time with this Wells, especially as we have no real idea how he differs to both the version who was Thawne in disguise and his Earth-2 doppelgänger. A third take on the same character in as many years may seem like a bit much, but if anyone can pull it off, you have to believe it will be Tom Cavanagh.