There's far too much competition out now, for them to bring the Geo brand back. The N. American market is even more flooded with compact/sport compact cars now. So unless GM decides to take all their compact cars, from all their different lines, and label them Geo's, it's not going to happen.
And they already have gotten rid of the Hummer brand. Hummer hasn't produced a car in almost a year now, May '10.
A company purely focused on fuel economy cars, just wouldn't cut it now (in my opinion of course). Like I said, there's just too much competition in the market right now. Everyone has hybrids or regular cars that have great fuel economy. Many companies are keeping prices fairly low for hybrids now as well, and are very comparative to regular gasoline engine cars. A company focused on just fuel economy (verse horsepower), would just miss out on a HUGE part of the market. There are just so many new buyers of economy cars, yet so much variety out there right now. It would just be too risky for a company to try something like that. Now, if a company tried it before the economy crash, when everyone was buying gas guzzlers, they might have had a chance to steal the market.
However, I do completely agree that a company needs to work purely on fuel economy (verses power). At the very least, the entire automotive industry could use a big push, to make cars with better MPG. Comparing non-hybrid car of today to some cars of the late 80's to mid 90's, it's an absolute joke. My own car ('94 Honda Civic CX ['92-'95 Civic CX]) is very similar to the Geo Metro (42-50 MPG/70 HP). The fact that ALL car companies can't come out with simple, quality cars, with that high of a MPG, is ridiculous. The fact that there are so many Metros and that generation of Civics out there, should tell the car industry a lot, but unfortunately they ignore it. Seriously, how it is possible that car companies can't do what companies did 15 +/- years ago? Honestly, it's the only thing that has stopped me from selling my car.
Of course, this is all just my opinion.