c and d are implementend in a satisfying way in 3b.

 a) This could be implemented in the "HEroes" tech but i am not sure: Champions should spawn in a tiered system like goody huts and quests, which should also be linked to the adventuring technology tree. Hireing champions of higher tier should also require the respective technologies.

This would allow to hire champions later in the game (where typically all champions are already serving the one or other souverein at the curent implementation). Therefore some advanced in the adventuring tree which allow cheaper champions will get more usefull. Furthermore it allows Adventuring realms to get access to more powerfull units.

Add some advances within the adventure tree (or other trees) which enhance champion - production abilities. E.g. you could research a "Writing Set" which then would become available from the item shop. This Writing set could have an effect like: "Increases Research and Spellpoint generation of the champion by 100%". Equivalent tools could be available for Merchants and all those other Champions. This would make these Heroes quite interesting during late game, while not requireing them to go out to level.

c) Allow Quests and Higher tiered goody huts to be only opened by Champions. This would reduce the need to send out as many "one-peasant-armies" as possible to gather all the goody huts.

d) If possible, goody huts, which are of vastly higher level, should be hidden from players without the required adventuring technology (or players which are two or more levels behind). This is mostly for convenience, as at the moment you visit every high level goody hut several times before you can use it. As these huts spawn, when one player reaches the required level it might of course be impossible to hide them.

I had thought of some more but somehow i can no longer remember them.