Maybe not, but perhaps becoming a Governor could be a way to "retire" a Champion. When one gets too many wounds or other disadvantages they can be transformed into a governor and cash in their levels and traits for neat city related advantages. They'd still be able to fight if the city got attacked.

I agree more could be done with Path of the Governor.  They should earn XP as fast as fighting champions and it would be nice if we had more solid Path of Governor traits to choose from.  In addition to the +1 research, +1 gildar, there should be stuff like:

Librarian:  +1 research for each research building in stationed city

Guildsmith:  +1 materials for each materials building in stationed city

Trader:  +1 gildar for each gildar building in stationed city

Physician: +5 heal/turn for units in stationed city

Barterer:  Shop prices increased/decreased 10% for selling/buying in stationed city

Diplomat:  City produces +x diplomacy capital for stationed city

Storyteller:  City produces +x prestige for stationed city

etc, etc, etc.

I think any hero with an ability which benefits either a city or your empire (ie + research, etc) should gain half way decent amounts of experience every turn if they are in a city. After all that hero has a trait which makes them good at doing civilization type stuff, they should be able to benefit from actually doing it!

I doubt that's going to change, but it doesn't really make sense, does it? What does a champ learn about governance and administration while killing monsters in a dungeon?

Ok, fair   I've worked in a bureaucracy long enough to have occasionally wanted to break out an axe. But that's got to be the exception, not the rule.

Sure, but that's not the bonus they're getting right now. It's bonuses to production.

Why can't all units gain xp every turn, period, like in MoM, but have xp requirements scaled to compensate?  Perhaps 1 xp per turn, but double the xp needed per level (or something like that).  This could be buffed by a skill in the Governor tree, let's call it "Training" that could grant say, 2xp per turn to all units in that champion's stack at lvl I, 5 xp per turn at lvl II, and 10 xp per turn at lvl III.  (Kind of like the trait some heroes had in MoM - I have no originality here!)

Passive xp gain would help the AI more than the player anyway.  The AI tend to have more units, and the player can often get hundreds of xp per turn with their champion stack by monster hunting.  Passive gains would buff the AI more than the player.  It would result in more mid level units - meaning fewer one-shot cannon fodder for the heroes - and a doubled or otherwise increased xp requirement would scale back some on the ultra-high lvl stack (which would now need twice as much xp for comparable levels).