Hi, Aerozol! I also got the Humble Indie Bundle, devoured Aquaria in about three days, and was inspired by it. Unlike you and Echolocating, I come from more of a programming background than an artistic one.
... which means that, like yourself, I'm looking for a collaborator from the other side of that equation. So, well... show me yours1 and I'll show you mine2! 
Hey Inyssius! Good to hear!

I've been messing with files in Photoshop, and getting used to using a tablet again (it's been a long time...)
I had big issues with the 'halo' effect too, covered another thread. But I suppose I'll just DL Gimp at some point, since apparently that fixes the problem.
To practice I decided to make a 'ninja' naija:

v0.1, v0.3
Which was a bit meh, but cool to get stuff working. I may fix/redo it at some point I suppose, or if people want it.
I also did a Pyramid Head costume:

Same Photoshop white halo BS, and I drew the whole thing too small, and had to stretch it up. So it's not exactly quality either...
But man, it's pretty funny to watch it swim around and flip and stuff : D
I have some ideas for a mod, but I don't want to ruin the surprises.
In any case, all the ideas are 'visual' ideas, which I think will work great with Aquarias set-up.
(Not mimicking its actual art style though. I don't think I can improve the original, and neither can I draw
that well)
Slightly off-topic, I also made icons for all the costumes/ mods I've downloaded, because it was looking messy..

Mmm orderly
And btw I have experience creating things like Logos, Icons, and designing game Titles, so I thought some people may be interested in that. A nice looking presentation can do a lot for a mod.
Anyway, it's been fun!
p.s. your turn Inyssius ; )