Right. Here it goes. **WARNING, WALL OF TEXT DETECTED**
I had been part of a Gundam MUSH a while back and their approach was interesting. Their story started at the first episode of SEED and used it's history as a base, but from that point onwards, the story was what the characters made it. For example, Kira never made it to the Strike and was killed, there wasn't a lot of focus on the canon characters (though some did pop up from time to time) and the story followed the characters, not the heroes of SEED. It was basically a re-telling of SEED in such a way that we shaped it ourselves, essentially creating an AU SEED. It worked surprisingly well and allowed the players to go in lots of different directions. IC choices had IC consequences that affected the world (Who was in charge, what battles took place where, etc etc). It was heavily focused on characters as opposed to plot; plot was still important, but it was the characters and their struggles that really made a lot of the RP scenes shine.
Another thing they did was what was already suggested with the customization; you got points that you could use to buy suits and upgrade them with weapons and equipment. Each suit/weapon/equipment had stats for rolled combat, but most scenes were done entirely as RP instead of a D&D styled combat round. I liked it because it added versatility and kept combat fresh. Each character had a character sheet as well for rolled combat or other events, but most things were RPed instead of rolled. These points were rewarded for completing scenes and RPing, and sometimes as options in loot drops.
As for player activity, they had an interesting system in place. Instead of creating an event and telling everyone to join, the admin would send out an announcement asking interested players to send them a tell/PM if they wanted to partake. They then incorporated those characters into the scene naturally. This ensured that the people in any given scene WANTED to be there. I feel RP was a lot smoother and rewarding as a result. Granted, that was based on real time RP as opposed to forum-based RP, so I'm not sure how well it would work in this scenario. Worth mentioning though, I reckon.
Loot drops could be another idea. At the end of scenes, the DM running the scene would make a list of loot (suits, equipment for suits, weapons for suits, character scale equipment like weapons or other vehicles, or even just points for suit customization, novelty items, character specific rewards etc etc) and the players involved would roll 1d100. Highest or lowest roll would to choose what they want. The DM would try to make it make sense in terms of the storyline as well, (Like if the scene is raiding a ZAFT station for example, the loot will be all ZAFT stuff that would be on the station naturally.) Sometimes, if the scene was a big one and it was well-done, the DM would give out specific character rewards that made sense for a character (Like if your military character was involved in a large-scale successful mission, they could get a reward of a promotion in their faction, a favour from someone higher up, or things like bases or buildings that they could customize and put to their own uses in the world). I really liked this because it helped characters establish and maintain a certain sense of uniqeness. A non-combat character and a military character could be involved in the same thread and get completely different rewards that reflected what their character was all about.
Not saying 'copy everything from this MUSH ever!' but I just wanted to point out some of the features that made it a fun experience for me personally.
In terms of the actual customization/battle/character sheet thing, I am unfamiliar with any system that we could use as my experience is pretty much limited to Pathfinder/D&D, Shadowrun, Mutants and Masterminds, and some homebrew Fallout stuff haha. Those systems are very number/math heavy though so my initial thoughts are that they're unsuited to a more casual forum-styled environment like this. There's a system called The Window that is a lot simpler but I'm not sure if this is the style we're really going for. We could just as easily separate number/math threads from the ones that we want as strictly RP, or even make up a homebrew system. I'm not good with numbers, I dunno lol. I reckon it would have to be relatively simple otherwise it might discourage people from wanting to take part in threads that required the system. Maybe just some base ability scores and modifiers, basic skills, and then stats for suits/weapons maybe.
I'd be interested in trying something similar with the storyline; set a start point in the timeline (like Heliopolis being attacked or something else), have an event to kick things off, and have the world and characters branch out on their own from there. It would mean it would still be an original AD storyline, but it would be familiar in the sense that it still takes place in the same world/universe. We could add or take away what we wanted (Canon characters/events, tech, etc etc) according to how our individual AD storyline progressed, and it would still make sense in the universe since the universe itself was organically created through everyone's collaboration. We could make it as serious or silly as we wanted and essentially just create from the ground up in such a way that it still feels like Gundam SEED, but it would be our Gundam SEED. Heck, we could even introduce AU versions of our previous characters if it feels SEED-ish enough haha.
This post is already really long so I'll wrap it up by saying that whatever we decide to do, I hope its in the vein that its still at least somewhat related to Gundam SEED as opposed to dropping everything and making it a more Generic Gundam RP. I reckon a lot of us came to SEED RPG and Advent Destiny because it was SEED related (I know I did), so it would be nice to keep that root/base.
Sorry for the wall of text.