In fantasy football as long as you have 8 guys to pencil in that starting lineup each week, you're competitive. You need to field a competitive roster every week of the season. With this in mind, if you want to add some spice to the league, add a transaction cost. Add that to your team entry fee and now you have motivation in the form of good old fashion greed. This all depends on your league.
Some leagues prefer auctions to employ salaries and create the whole keeper league concept.
Player salaries and contracts with 3 or 4 years in length and so on. My preference will always be the keeper league with an auction, rookie draft, salary caps and player contracts. That's the most fun, but also a little more intense for the average roto player. The point is you can run a rotisserie football league the same as a fantasy football league. To get really crazy, you could even run both from the same set of players and owners.