Well, Decay really needs some sources of actual damage. I'd give him some Obsidian Dragons.
Neptune shouldn't have Poseidon (Longsword would be better for him!) He should also have more ways to make use of the air mark he has. All he's got that I can remember is Eagle's Eye and Shockwave. Elite Wyrm or Sky Dragon might be good for him; plus it would provide ways for the player to actually win upped versions of those cards.
Seism shouldn't have Pulverizer either; he can't use it.
I realize you said not to change the mark, but I think Divine Glory would be stronger (as if he needed it) with a Fire mark and straight light towers. Usually when I beat Divine Glory it is because he didn't get any burning towers and so couldn't play explosions.
Any FG with two different types of weapons or shields should not have them. For example, Fire Queen will always prefer Eagle's Eye over Fahrenheit, and Miracle always plays Morning Glory over Jade Staff. I'm not sure if any FGs have two different shields in the same deck, but I see this as a common problem in the arena, where the AI will waste shields or refuse to play better ones than the one it has in play.
Wow... I just looked at the FG lists and Neptune has FOUR inundation... times two :p He doesn't really need those; they just clog his hand, as there are probably less than 1% of games where there are enough creatures for inundation to matter. He could keep one just for thematic reasons. He also has Poseidon and Eagle's Eye, and as I mentioned already, he doesn't need Poseidon since he has no earth quanta, and won't play it anyway if EE is out. (which makes sense, as EE is better for him, but Poseidon in the deck at all makes no sense except for theme)
Obliterator is already really mean, but Black Hole would make sense thematically for him.
Osiris needs more gravity cards to use that excess quanta on. Something like Overdrive for his scarabs would be cool. Otyugh would fit the theme, but is really unneeded with scarabs.
Paradox would get a huge benefit from Fractal. He always has spare aether quanta, and it would be mean as heck on cheap deja vus. And paradox is one of the weakest FGs, so helping him out would be a good thing.
Rainbow is already mean as hell, but he always seems to have loads of excess quanta, so something that could make use of it would help. Dissipation Field would be bordering on the ridiculous... Any other pumpable creatures, some of which he has already, would work well for him.
Scorpio using Poseidon is ridiculous. Also, I don't think he should use Purify, because it gives Mindgate decks an answer to defeating him with his own cards! I understand people might want to be able to win upped Purify, so give it to Neptune instead; that would make just as much sense.
Serket needs some steals. Shields are his biggest weakness. He also doesn't have any shields himself; maybe Skull Buckler would help him? Aflatoxin or Grey Nymph would fit his scorpion/poison theme, probably a lot more than flesh recluse does. Mind you, the recluse is a good card, it's just thematically weird for him.
