I just read the parts about TB in FotR. It says he was not affected by the ring BECAUSE he did not want power at all. He also could not control nature, or the trees of the forest. This really doesn't fit with him as a ring hero. The Ring wouldn't give him any extra power, nor does he want it.
To be honest, with all the good factions you'll have the problem about the ring heroes as pretty much all the heroes who could have wielded it rejected it (Aragorn, Galadrial and Gandalf to name but 3). Okay, the idea of a novelty ring hero is appealing (something like Samwise the Brave) but possibly not the most powerful or useful (Sam against Sauron? Obviously too weighted in Sam's favour). If it is a quick fix, then Tom probably is better than a corrupted Galadrial though (hey, if she becomes corrupted why not go further and have an evil, all powerful Gandalf for men?)
Do like the idea of the good factions destroying the ring, either to make it game over or to cripple any of the evil factions. Another thought though, what about the ring "hero" being the remaining members of the Fellowship for the factions? So you get the ring back to your base, and after the timer ends you have under your control the entire fellowship of the ring (with Frodo holding onto it) with all the members at level 10 and, if they are reasonably close to each other, with a heavy attribute increase so you are rewarded for using them as a group rather than just 9 random heroes (they did fight pretty well together). Of course, doing so is reasonably risky as Frodo isn't the strongest hero out there and there is the real possibility of him being killed and you losing the ring.
Still has problems though. It makes the Men faction's ring hero very weak (wow, you get 1 elf, 1 dwarf and some hobbits...) and it still feels very much second best compared to Sauron (rather than close to even). Still, it avoids having a corrupt hero on the good sides and gives the player a group that would be very hard to defeat (the hobbits throwing rocks, Legolas shooting arrows and the resting fighting the enemy with swords/axes, keeping the orcs off the more vulnerable characters behind) whilst keeping it closer to the books and film.
Sorry if this has been said before, but I really don't have enough time to read through 21 pages on a thread.