I say 3-5 keepers max, 1 must be a rookie you drafted, you lose a 1st, 2nd, 3rd round draft pick for every keeper, 3 years max then the keeper goes back into the pool. If an owner leaves the league all the players go back into the draft pool. The new owner gets an extra 1st and 2nd round pick (in lieu of late round picks).
I like the gist of that. Especially hitting on the player leaving since that's a yearly occurrence.
I like the idea of 5 keepers with tradeable draft picks, tit for tat (ie I trade you a first round pick for top talent, you trade me a last round pick and I drop a player or trade you a lesser player).
Also, I think the draft for the following season should not be loser drafts first, but the bottom 5 are randomized and the top five are randomized separately. This will create parity while also ensuring people don't just tank at the end of the season for draft position.
I am not sure what way I like best for draft position forfeiture with a keeper. I like the idea of 3 years max per keeper, I personally like the idea of being able to draft a 6th round player and be penalized one or two rounds for keeping him. Or even better, finding gold in an even later round. This would reward forward looking players and ensure some top picks still available, rather than everyone just keeping the top players every year until three years pass. We'd have to have a rule for keeping players with only top rounds ahead of them, maybe losing your first round plus something ???