1. Double-shot keycaps
You can always buy new keycaps, but you would probably need to either find a Cherry with doubleshots on ebay or import a Poker with dye-sublimated PBT keycaps if you want nicer keycaps to come on the keyboard. Or spring for a keyboard with Topre switches, since I believe they all come with dye-sublimated PBT keycaps.
2. Backlighting
Backlighting and good keycaps don't mix. If you really need backlighting, I think the Ducky Shine currently sounds like the way to go, but you're probably best off dropping the backlighting requirement if you can.
3. Best switches for combination of gaming and typing, 75% time spent typing, 25% spent gaming. (Cherry MX Brown? Blue?)
It really depends on individual preference, but Cherry MX Browns are the safe choice here. Like a lot of people, I'm a fan of blues for typing and reds for gaming, but browns are good all-purpose switches.
4. Ergonomics
Unless you're willing to try a more exotic board like the Kinesis, you should probably just get a tenkeyless board if you're concerned about ergonomics.
5. Available with PS/2 interface.
All(?) keyboards on the market these days with Cherry MX switches have USB interfaces, but many will come with a PS/2 converter, which should be fine.
6. Appearance / Aesthetics
This obviously depends, but a lot of people here prefer the minimalist Filco/Leopold look. The Filco has more angular edges and looks more serious, while the Leopold has rounder edges which make it look a little more toy-like.
7. Cost
Unless you're willing to find something used or seriously compromise on some of your other requirements, you'll be looking at $110-140 for a decent keyboard with Cherry MX switches.
8. Features (USB hub, media keys, etc)
The Das Keyboard can function as a USB hub, but is this really something that matters much? I feel like most/all computers these days have a number of easily accessible USB ports on the front, and it's easy to use software like AutoHotKey for media functions.
If the possibility of legend-wear is the main reason you're interested in doubleshot/dye-sublimated keycaps, you might want to consider the "Ninja" Filcos that have the legends on the front of the keys instead of the tops of the keys. They're not the cheapest of keyboards, but Filco is generally regarded as the top of the line brand for Cherry MX switches, and then you wouldn't need to get separate keycaps.