My #1 feature for a year round league: Keep a free version! Or else a lot of keeper leagues would just stay away and just start a new league every year and transfer rosters over using free leagues. Start with free year round leagues that only differ in that the league doesn't vanish and can be reused year after year (along with the minor tweaks that need to be made to make the transition from offseason to the season work. example: ability to change team owners which is disabled after a season starts in Yahoo leagues), THEN a whole bunch of nice bells and whistles can be added for some special year round leagues that cost money.
Reading this thread a lot of the ideas sound to me like they don't necessarily have to do with the idea of year round leagues as much as they do Yahoo leagues in general.
Features I'd like to see include:
All-Time stats/standings, (example: standings that not only include the current season, but past seasons as well all into one). That would work great for h2h leagues, but wouldn't be too bad for roto leagues either.
A automatically managed method for doing slow, offline drafts within the league. Using a message board works ok, but if there was a method which made the commissioner's job easier, then great!
And most importantly the ability to incorporate minor leaguers into the game. That way, if someone for example owns the rights to Alex Gordon, they don't have to wait for him to be added to Yahoo. He'll sit on that teams roster using up a minors spot (or something to that effect) and not be usable until that team makes room on their major league roster and activates him. At least one person already mentioned this in the thread, but it's just that important that it needs to be mentioned as much as possible.