I recall somebody bringing that idea (players only being able to play in 16 games if the league goes to an 18-game schedule) a year or two ago when Goodell and the NFL first started talking about an expanded schedule.
On one hand it would make for some very interesting coaching decisions. Does a coach opt to keep a couple of starters out of each game? Does he leave several starters out for one or two games? Leave as many starters out as possible against a tough opponent on the road?
On the other hand what about fans that bought tickets to a game, or maybe paid good money on travel and hotels to see a team play - and then they sit their best player(s)?
And then of course there is gambling angle. Right now teams don't have to announce who is inactive until 90 minutes before kickoff; does that stay intact or does a team have to announce it's "two-game off decisions" earlier in the week? Some inside knowledge on who a team is going to sit could have a huge effect in the betting world.
It's a very interesting consideration, but one I doubt we will see implemented.