I would love to have a widget that showed the games I'm playing, and is able to look at my Steam, XBLA, and PSN accounts. There are currently widgets out there that will show your Steam OR XBLA OR PSN gaming history, but there isn't one that will show them all in a single widget. In order to show them all, you would currently need to have three different widgets on your site, all with disparate designs. Tacky!!
I imagine this unified gaming widget would show the last five or ten games you played, most recent at top. Personally I'm not interested in displaying achievements or scores or anything, just the games I've been playing. I imagine it would look something like this:
Bioshock 2 XBLA 15h
Portal 2 Steam 6h
Journey PSN 1h
Mass Effect 3 XBLA 22h
I'm guessing the reason this doesn't exist is because it would probably require learning one's way around three different APIs, which is more work than anyone wants to put into something like this. Still, if someone feels up to the challenge, and they execute it nicely, I bet a lot of gamers would love to have something like this!