Warren ... re: time limits, obviously the longer the better ... though I am never in favor of a drop dead or drop dead revert back. I agree with Crystle re: finish the inning plus one, though I know that has the most chance of playing havoc with a TD's schedule, so finish the inning is the next best thing short of a full game. No game should go only 2-3 innings ... I have been involved in way too many 6-4, 8-2 games (not low scoring, but not run rules either) over the years where the teams only batted 2-3 times ... sorry, but that's just not a full game. Would any basketball teams be OK playing only one half?
I like double elimination personally as well ... as it gives the best teams a better chance to win (i.e., they can have one off game), and all teams a second chance.
The format you mentioned above would be attractive. I've coached at all age levels, and don't see any strong reasons why age levels matters for format.