well, the japan launch date is only a few weeks away anyway (it releases in japan one day before my birthday!) so hopefully we'll find out the final roster sometime soon, and isaac better be on it XD
as for the FS, i think that megiddo wouldn't really work as one, it'd probably be better as an up+b move or something... best FS for isaac would probably be judgment, but really any one of the higher level summons would work fine. eclipse, daedalus, catastrophe, charon and iris would all be great as an FS, i think.
i think it'd be cool if isaac started off with only judgment as his FS, but you could find and pick up summon tablets while playing that would add another summon to his possible FS. then, when he uses a FS, the game picks a random one from the list of summons he's found and uses that. the effects of the summons would basically be the same, with a few slight differences between each one (like iris would heal isaac, charon would have a chance of instantly KO'ing an opponent, etc).
or summon tablets could be an item similar to assist trophies? O.o pick one up and see a summon devastate the field for you... but that'd probably be a little too powerful.
oh, i was trying to think of a good item from GS that could be in brawl, but at the moment all i've come up with are the catch beads which would act a bit like the franklin badge. i was thinking maybe if you have the catch beads, if somebody throws a projectile like you, if you shield when it hits the catch beads will activate and it'll show the psynergy being used, and you'll end up holding the item. this would be useful for things like bob-ombs, mines, pokeballs, etc that activate after being thrown once, it would prevent it from activating and let you use the item instead.
wow long post, and what happened to the new thread?
edit- oh, i just had another thought. what if isaac's FS was judgment, but you had a small chance of getting catastrophe instead? they're very similar summons (at least in terms of appearance), so it would basically be the same FS, just stronger.