1) My favorite heroes are Sauron, Shagrat & Gorbag, Theoden, Bilbo, Witch King, Boromir, and Saruman. My reasons being that all of these heroes are pretty useful, though some are harder to use than others like Saruman and Theoden. But I also love heroes that have a unique incorporation with their factions such as Sauron and his influence of Sauron ability that makes your army stronger, or Theoden's ability to empower your armies. The best part is the ring heroes, I can't tell you how satisfying it is to control the battlefield with ring hero Boromir despite how I was outnumbered by the enemy. Or the crazy variety of abilities that Ring Hero Sauron possesses.
2) In terms of heroes I don't like or rarely use, I can think of a few. For example, though I do like the concept of trolls, Mollok is a bit too expensive and difficult to use because of how weak he can be in combat when fighting archers and spearman, despite how he's suppose to be a tank. He is decent in the late game but he isn't that useful in the early game especially compared to the Nazgul, I would like to be able to use him more if he had more utility with Mordor. I always found the scout heroes to be very boring with the exception of Shagrat and Bilbo, the Spy from Bree is ok, the big reason is that the other two scout heroes Merry and Pippin are used mostly to scout the enemy and provide visuals on their progress, which I feel has no significance in the meta game itself. The better scout heroes provide good economic benefits or cheap combat aid. Such as Bilbo's gold from the mountain ability or Shagrat's versatility. I admit the spy from bree has his uses in terms of his ultimate ability granting you resources, but beyond that all of his other abilities and pretty boring and not too useful. Merry and Pippin feel flat out useless in the end game. As for non scout heroes, I feel that Isengard's heroes aren't too useful, Saruman's tower mechanic is pretty cool despite the small range of Saruman's abilities while he is in the tower but other than him, the other Isengard heroes don't seem too useful to me, especially since when I play as Isengard I try to devote my economy to upgrading my units, and I feel that the heroes don't compliment that tactic as well as they should, especially Grima who seems too situational. The Rohan heroes though useful, they don't seem that fun when they are simply meant to compliment your army, rather than being useful on their own. But maybe that's the point of the Rohan heroes. Not sure
3) In my opinion the heroes are a mixed bag, on one hand with game changer heroes like Sauron, Theoden, and max level Boromir, you feel accomplished for the work you did in leveling them up. And showing off their powerful abilities to destroy the enemy or empower your army makes you feel all the more accomplished. Some heroes though don't provide enough utility for their cost like Gandalf, Saruman, and Mollok but have the potential to be pretty fun to play but their lack of utility or combat strength makes them less fun and more repetitive to use.
4) I feel that some heroes should be tankier like Mollok, Gandalf, and Gothmog. My idea is that heroes, especially the more renowned heroes from the lore should be able to hold their ground, not against large armies but small groups, and get taken down should they get overwhelmed by very large numbers or small groups of upgraded troops. This shouldn't apply to every hero but tankier heroes should be able to take a lot more abuse than they currently are in the game, especially damage from archers. Considering how insanely useful archers are in the game I think they shouldn't be able to deal as much damage to heroes as they currently do in the game. More specifically, heroes shouldn't be as vulnerable to infantry but rather more vulnerable against enemy heroes. In a nut shell, the health of heroes is not as healthy, I am not saying that for all heroes in the game, I am just saying it for most heroes.
5) I do like the melee damage of most heroes in the game, but it would be nice if heroes could do more damage against Calvary to discouraged spamming those units. I guess I just find it annoying when a hero is being chased down by calvary and can barely defend himself even at higher levels, I feel that it should be discouraged to use calavry against heroes and instead use upgraded soldiers, pikeman, and elite units instead. Other than that, melee damage is fine.
6) In terms of ranged hero damage, I feel that range heroes are simply meant to deal damage from a distance so they are less likely to get killed. I rarely use faramir but as for Lurtz, I don't switch him to melee mode except as a last resort to prevent him from getting overwhelmed. I would call the melee/range switch function an ok concept. Not great, but ok, I would leave it.
7) Gandalf's spells are definitely too weak for his cost, I feel that he would be a lot more useful and fun if his spells were more powerful like in the older versions of BFME, on top of the fact that Gandalf is pretty weak health wise.
8) In my mind the Wizard blast should be used as a crowd control ability to force the enemy player to manuever his armies carefully, but in the game the ability is pretty weak against non-basic infantry units, like if I hit a troll it would only damage like 1/3 of its health rather than push it back or increase the radius of the ability. The ability should make Gandalf less vulnerable against crowds of enemies to make up for his low armor. As for the damage question, I would say 1500-2000, since only the elite units should be able to withstand such a powerful ability, as I stated earlier, upgraded or elite units would be the preferred non-hero counter to heroes. If his wizard's blast cooldown needs to be increase with this buff than I am all for it.
9) As for the Word of Power. I feel that giving a 3000 cost hero a game changer ability is the key to securing his role as a game changer hero. Increasing the damage seems very necessary to making the hero useful, of course there are ways to prevent this ability from being too op such as making the cast time longer so your opponent can react to it more easily or having the ultimate destroy non-upgraded units but heavily damage upgraded units. This also applies to archers, siege weapons, calavry units, and monsters. But not being too effective against enemy heroes. So overall I would prefer if the ability was more powerful but with a higher cooldown, casting time, and damaging specific non-upgraded units. Making it the perfect game changer ability for Gondor's game changer hero Gandalf.
Sorry if my suggestions came off as confusing or unclear but I hope these suggestions on hero improvements help improve the game overall.