The best buttons ever, btw. I like em better than Sanwas myself.
As for art design, I originally thought I'd do the same sorta thing with Hilde, with her in the front, the rest of the cast, typography...but you really need it all there? Really, most people are going to notice one thing; the picture of Ivy in the center. There's not a whole lot of negative space on an arcade stick. If you've got just a picture of Ivy on a white background, don't waste your time finding a small picture of the SC4 cast, cutting out the edges on that. It really depends on what you want for your background; you want it more white or black? If more white, you can save yourself a lot of time cutting off the edges on your main pic.
Also, do you want to put button sillhouettes INSIDE those buttons, like light gray X, Triangle, O, and Square buttons?
Oh, and an idea for "text" to put against the top and bottom edges: some of Ivy's combos. Hell, even CS or SS would be funny. Someone who doesn't know SC4 asks you "that's a nice arcade stick...wait...what the hell is along the top and bottom?" and you can chuckle at them a little.
As far as backgrounds, something I like to do is find a real background somewhere (nature sites, maybe an anime site for an Ivy-looking cathedral, architecture site for a real cathedral) and apply a ton of filters to it to make it look completely different. Desaturate it, filter it, level it, (bop it, pull it, twist it!), etc. I'll attach my example.
