We're all aware of the Sword-Thain card, and I've thought occasionally about what would be the best unique-ally-to-turn-hero if you had a quest that allowed you a free sword-thained ally to start.
But conceptually, any ally, unique or not, could hypothetically be made into a hero with the same or invented name, with a threat equal to the sum of stats and the ability on their text. In some cases that ability would be useless, or even disastrous (Escort of Edoras). But in some cases it would work exceptionally well -- Yazan, Jubyar, and Firyal would make *excellent* threat-9 heroes.
If the entire world of allies were open as heroic options, what would be the best three-card lineup consisting of:
1) An actual hero
2) A unique ally
3) A non-unique ally (assign an invented name).
I'd kind of like to play this combo -- Thurindir (threat 8), Arwen (from Watcher in the Water, threat 5), Vigil (Vigilant Dunedain, threat 9). Starts at a low 22 threat with a side-quest in hand, Arwen can boost Vigil to 3 defense by questing, and once he has some defensive attachments and a side-quest completed he'll be a monster.
In general, making allies into heroes would provide a wealth of secrecy options, while the current pool is very constrained in that regard.
Some other non-unique allies I think would have real potential as heroes are Mirkwood Runner (5 threat), Erebor Battle Master (4 threat), Mirkwood Explorer (5 threat), Rider of Rohan (6 threat), and the upcoming Emyn Arnen Ranger (4 threat). Some other unique allies I think would be good would be core Faramir (8 threat), spirit Elfhelm (8 threat), leadership Erestor (6 threat), Forlong (6 threat), Anborn (8 threat), neutral Treebeard (14 threat), ally Galdor (9 threat), Azain Silverbeard (7 threat), ally Boromir (9 threat), ally Gimli (9 threat), ally Legolas (8 threat). The upcoming Elladan and Elrohir allies, as heroes, would make a wider variety of twins deck much easier.