interests :
-linked to the "hero" promotion + grigori civ so that : captured heros from ennemy civs can be converted by Cassiel into being adventurers for the good of the grigoris (they lose their unique abilities that were not promotion related but gives you a new adventurer: the grigori player has a tough choice here ) +grigori "hero" unit captured by ennemy cannot revert to adventurer.
-can optimise your promotion path, transforme your high experienced dragonslayer (but low level as you converted few xp into levels) and allow him to use all his free xp to become an invincible longbowman (Robin hood) with every archery related promotion you can get...that can stop hords of barbarians ; get an high mobility, high evasive action archmage : upgrade throught the mounted line to get mobility II and tactics III then begin anew with the robes of the adept to learn some spells.
-get "some" mundane soldiers that can launche some spells : former mages/adepts that converted into the melee line or archery line.
...etc
-reflects a bit the DD system where in some cases you can get adventurers with mixed specialities...
I know there was already such a proposition (maybe for another civ: shapshifters) and there were issues but I may have
some solutions !!:
-to refrain from upgrading in the arcane path many times gaining 1free promo for adept then mage then archmage then back again (or worse : adept then adventurer then adept, cycle again : many free promotion):
make it so that the an adventurer with chanelling 1 cannot upgrade into the arcane path. Once you went up to adept/mage/archmage : make the best of it as you may never go into this path if you leave it.
-maybe
make it so chanelling 3 forbids going back as an adventurer (to not have über arch-mages that can launch many lvl3 spells but are immortals as you upgraded them into the melee line learning all useful spells.)
-to limit the abuse :
"going back to adventurer" costs 1
per xp or 2

/xp. It would then be expensive: if you change lines many times it may cost you a lot (a mean of 100

for becoming an adventurer + some gold to go upgrade into units of said line). You could add +5gold per magical related promotion (mana promotion, spell enhancement, twincast) : A mage, on the verge of attaining omniscient power and infinite life by becoming an archmage-lich has to be paid a lot to abandon his studies and learn new tricks as a n00b in another branch.
-
instead make it 1
per xp -3gold per level.... each promotion that has been used being a commitment, the adventurer may be more easily convinced that he has learned all the tricks of the trade and should explore something new. It also is a tradeoff for gaining huge amount of xp in attack and not using the xp (ex with an immortal) then going into another trade to get many promotions specific to this trade (recons' sightII/III, subdue animal/beast, ranged's archery promotions + precision, arcan's many mana promotions). If you have spent xp for some promotions before becoming again an adventurer, it is as much xp you won't be able to use for specialisation in the new trade (so I think there should be a compensation for the player : reduced cost)
-if this ability needs movement,you won't be able to convert religious heroes as afer being "commanded" they have no movement and in the interturn they leave you as you have no religion.