These are the words I live by as commish, and you should, too: Don't get cute with your league.
You want to track Fielding % as a scoring stat in your league? Fine, a lot of sites support that stat, but good luck finding any analysis out there that takes it into account. Almost all preseason forecasting will be useless. If you guys are all that hardcore, great, but you're more likely to wind up with a bunch of frustrated managers who will quickly lose interest in the league.
Your "dual catcher" idea sounds weird (it could drastically affect the value of certain catchers, depending on the backup), but you could probably "make it work" by having a 2-catcher league, and a House Rule (because no site would enforce it) that both catchers have to come from the same team. IMO, a playing time is part of fantasy bb strategy.
Lastly, be very careful about changing the rules of a keeper league. Once you start, any significant changes you make should have a full season of play before they take effect. So, I would not plan on any major changes taking effect until the 2014 season, because people (hopefully) drafted and are playing already with an eye toward 2013.