If you want to avoid the pitfalls with GEM and GC, please focus work on the synchronization of events. We have extensively tried out both GEM and GC for several months each and we ended up using PHPRaid, basically because the calendars do not work in a reliable fashion.
The most common problem was the following:
Officer creates event. Players A and B are online at the time.
Players A,B signs up to the event.
Officer goes offline.
Player B resigns.
Player B goes offline.
Officer comes online, event is starting, officer sees Player B still as signed up (even if player A could have relayed the information about player B resigning).
We had huge problems with this all the time, both in GEM and GC. People resigned when the leader (me) wasn't online and the synchronization when I came online had major issues (mostly when the players who had resigned weren't online at that time). All poeple in the guild had the mod installed, so all were able to relay the information about changes. Basically noone ever resigned when there was no other person online (which of course creates an unsolvable problem if you don't have an external database for it, which it logical). There was always some people online at the time of certain resignations and at the time when the leader came online.
I know this synchronization isn't easy and I've been working on it myself, but if you really want to avoid to be "just another of those calendars", put a lot effort into the synchronization :)
Please don't limit Raid management to the guild! On our server we have many Raids with members of several guilds. At the moment we use GEM, but something new and clean is definately highly appreciated.
I would love an alternative to GEM. We raid with an alliance of 5 different guilds, but GEM had so many bugs with people's signups being dropped and moved into sub queue for no apparent reason, that we abandoned it as anything but a "hey, go to the website and signup for x on x date on our phpraid page" in-game notification, and cross-guild chat channel.
I for one will be keeping an eye out for updates, whenever you get to them. :)
A support for GC and GEM will be great, that way we don't have to force all guild members to choose this addon over the other. That's what I love with most Ace/Ace2-based addons, they provide support for other similar addons.