I think as a proof of concept, to show that this sort of thing would work and that there's demand for it on WA, you should start small and do the first season as a cup or just through a forum thread. Have between 4 and 8 people volunteer to be captains, between 12 and 24 other active and easy to contact (non-captain) players sign up for it, then do a draft. I don't think it's that important for the teams to be evenly matched on paper because a lot will depend on team synergy rather than sheer skill anyway, but if you're worried about that, just go with fewer captains (so each captain definitely gets a solid first pick) and a tournament system in which the worse teams aren't punished severely (in turn losing all hope after only a few matches played).
A system that would likely work well for this, if you were to do it cup style, is to start off with a single group stage in which each of the teams plays against each of the other teams that signed up in standard clanner format (one or two league scheme picks a side). 3 points for a win, 1 point for a tie, 0 points for a loss. After the group stage, the top half of the standings table advances to the upper bracket and plays against each other, same with the bottom half of the group stage in the lower bracket. This way, no team is out after the group stage, but doing well during it (securing a top 4 position in the case of 8 teams taking part) means that in the knockout stage, you have to lose twice before you're out. Which is nice.
Something like that should result in good competitive fun, not interfere with the clan league too much, result in less drama because there'd be no avoiding, as well as give everyone an idea of how much interest, activity and organisational trouble can be expected from a format like this in the future. Then for the next season, either keep the same cup style format or look to expand into a time-limited league with playoffs. But if you ask me, there's too many of those on TUS already.