For fantasy baseball the past 3 years we have been using yahoo (head to head) but after now using myfantasyleague.com for fantasy football, many of the league members want a more similar experience. We just started doing keepers for football and have really enjoyed it. A few key componets of our keeper set up,
Whoever you keep costs you a pick in the round before he was drafted this year. For example, if you drafted a player in the 12th round and wanted to keep him, it would cost you the 11th round pick. Also you only keep guys for the most of 3 years. It is important that I'm able to set which player is taken in which round for the keepers.
Another important feature for us is being able to split up the league, 14 teams by division. For football we did 7 and 7 and the top 4 teams from each league play in the their own playoffs. The winner of those playoffs plays in the championship.
Finally for baseball we want to be able to set a minimum innings pitched for each team. We want this to be a head to head league as well. Any suggestions?