I'm not sold on having to map to fuel your cart and such. Also don't see it as a viable leveling alternative because of limited fuel. Overall, while I think fuel fits well for a temporary league mechanic that's not meant to replace the content, I feel if/when they introduce it to the main game, they should probably think about making it so you can buy fueld with azurite. Do well in Delves, keep playing Delves kind of thing. Hell having a way to make it an alternate leveling path would be great too. People have been asking to map from lvl 1 but I feel this would fit better since it's a separate system that already scales down to lvl 1(or whatever, start in mud flat or the coast or some shit so lvl 2).
Maybe a toggle on char creation like SSF, unlocked only after one of your char on the account has cleared say Shaper or whatever(to not confuse new players). If you start the char as a "Delver", you can't do any of the story quests, you have unlimited fuel until lvl 68 or whatever level the story stops at, you get rewarded books at certain tiers, for certain rooms or for buying them with azurite so you don't miss out on these. Gem rewards for specific delve clears.
Would need some work to implement well, but I think it'd be a great way to give people who just want to kill shit rather than look for the next exit a change.