Now that's nice Adventurers Guild. Covers a range of imagination and is useful. i'd race to this wonder sometimes.

'Senate House' should be a more neutral name, because Kingdoms and Empires, and be a world wonder.

-10% universal Unrest in a building you're guaranteed to get translates into "take Rebels or Cruel for the free points", with Cruel already being considered practically free already.

Well it's a high fantasy game so the buildings could have fairly ostentatious names: The Council of Lords, The High Moot, Hall of the Highborn, House of Nobles, etc. Good part of fantasy is that the naming conventions is so over the top.

No matter what the xpt gain is, it will be either overpowered at low levels or useless at high levels, UNLESS it's tied to the hero's level. 

or Grand Council

or Realm Grand Council...

Is that necessarily a problem? I certainly think low level heroes should get better benefit from being in an adventurer guild than high level heroes.

Some partial scaling could be an idea so that high level heroes still got a noticeable benefit but I would expect it to be overall less useful still.

Barring some of the drawbacks mentioned by folks above, I think your idea is off to a good start.  I'll be looking forward to seeing what you come up with.

Sounds like a question for some master modder.  (You all know who you are.)

In their absence I will offer a definite 'I think so'.  Seasonal healing for units works on a per level basis and buildings can provide xp per turn.  Not sure of what difficulties might be involved in pulling the two together though.

An alternative I like for the Adventurers Guild would be a (very) low chance to spawn a hero for recruitment each season (maybe 1%).  Maybe that is too random but it would make it well worth getting the building (to my mind) as soon as possible in a game.  Certainly not something you could count on though, or plan a game around.

Someone else had the suggestion in another thread of instead having the occasional event pop from the building where a budding adventurer spawns and gets the chance to become a hero if you wager some resources on their success in a mock-quest.  A few alternatives could be given from losing the staked resources (gildar or other) to making a minor gain of some sort or getting a base level champion as a result.

I feel the latter alternatives are more fun mechanisms than getting +x% and might fit better with the FE gameworld.