BugReportage: FFH2 is mainly designed for singleplayer, and there is an option do disable lairs. So just use this if you want balanced multiplayer. Not all that hard really (some players also seem to disable huts, events and barbs or features for that.) + in coop it should be more fun with harder lairs and coop is viable multiplayer as well. Isn't it?
Agreed on the experience and certain promotions (like hero) should give better results. But i belive that is allready in. AT least it was in the modmod.
But if you can, give Marnoks new Modmod a try. You'll find some mechanics there to handle the midgame-issues you did describe...
@ Marnok:
I really hope your modmod will feature at least some kind if the old big badass power again and the possibility of monsters to build lairs again so more get placed over time and they are much harder to clear completely (still missing deap-sea and peak-lairs

At least monster-spawning ocean ones should make their way into the main-mod imo. To make the seas a bit more interesting finally. Not just the coasts...).
I definately miss those City-Raider 1+2+3 Water Elementals ~ Turn 90. Makes you watch out and really struggle for survival.
Those normal pesky Ogres are rather meh... just needs 2 or 3 experienced warriors and some strategy to handle

(Still by far better than before naturally. Don't get me wrong. Im really really glad that the explorable lairs have found their way into FFH2 main game.

And i understand that some kind of balance was needed...)
Oh and i miss those rare flying privateers / ships as well. Some real incentive for boarding parties to go for a little bit of swashbuckling.
But overall i agree that lairs should be a tad bit more common even in the core game (if lairbuilding by barbs won't find its way into the mod that is...).