Hi, let me first apologize for addressing this question so late. Currently, our plans are to hold sign-ups on AO3, and to the best of our knowledge, we don't think it's possible to configure the AO3 sign-up form to handle different sets of eligible fandoms, short of holding separate sign-ups for different forms of fanworks, which would not allow people offering fic to request art, etc.

The fanwork data collected so far show that there are less than a dozen outliers that show massive discrepancies between numbers of fanart and fanfic (at least, with the data we have so far and with the tentative cutoffs we've been considering). It should be possible to just allow these fandoms through despite the fact that they are eligible under one form of fanwork but not another or perhaps allow the community as a whole to vote on them.

I'm not categorically ruling out the possibility of running sign-ups outside of AO3, but it would require me (personally) to code the sign-up form and matching algorithm, so I would have to weigh how much time it would take vs. how much time I could spare. If people are really insistent that they would like to see this feature, where different lists of fandoms are eligible depending on the type of fanwork being requested/offered, please comment and let me know so I can take that into consideration.