My advice would be to make it uncomplicated -- the lower the threshold for participation, the greater number of participants you'll have (though perhaps at the cost of quality participation). Voting is handled pretty easily by the forum's polls, encourage people to explain their reasoning but I wouldn't make it mandatory for a vote to count or anything. Anonymous submissions are an extra step IMO.
For "how it should be done," I wouldn't feel bound to the Fill in the Blank format of making a coherent set either. By all means if that sounds best to you go for it. You could also take it one card at a time and have everyone build around a particular theme/keyword/card title/whatever. Some threads in the past were to make smaller, weaker subcultures stronger (Sauron wraith cards are a popular target) or improve the balance of formats (by adding new cards of course, not by editing/banning old ones). You could also do "runs" of cards, Decipher often took a formula and applied it to every culture for a different effect (e.g., the "remove 3 to prevent" FP events,
Tidings of Erebor through
Hobbit Sword-play). Or something else! Any sort of constraint is enough for creativity.
I think 20 cards is too many personally, takes a lot of devotion from yourself and any participants to do anything for nearly 6 months
But I can't commit to being an active participant either way.
Some people really only want to tinker with new rules/mechanics/keywords, others really only want to tinker with existing ones, and of course others don't care much either way. As a member of team "existing things" I would advise not being firm on one or the other unless the new/existing rule/mechanic/keyword is the point of that particular contest. Not sure what you mean by "premium" characters or modifiers.
It'd be nice to get some life back to these boards. Maybe they're really dead, or maybe they're just sleeping. Whether this takes off or not, cheers for the attempt. Good luck!