Well, natural yeast stops making alcohol once 11-13% concentration is achieved. While normal US beers are at 4-5%, there are some Belgian beers with 8-10%- and they are good. If one could concoct some special yeast strain working up to, say, 20-25% alcohol without producing unpleasant side products, it would improve beer and revolutionize wine-making (just imagine: natural wine will come already fortified!!), besides halving the amount of fermentation waste in ethanol production.
I'd like to see a lager yeast strain that would ferment at 65-70 degrees. You can make a steam beer that way but I'm talking about a true lager.