We're pondering trying something - a new feature. (And some marketing talk.)

[mdboucher]

Allowing anyone newly brought in to participate in building their team seems like a huge anchor to keep their interest. Things that encourage more involvement--trading and free agents during the season, for instance, would also be big for retention, I'd think.
One other suggestion for recruitment is that part of the draw of fantasy football is playing with your group of friends. I have no idea the feasibility, but allowing registration of a group of people to play a league together seems like it could bring in others. You could combine a couple 4-8 team groups to fill out a league, for example. Engagement would be higher if it's your friends that you're trading with.