Thinking about it, beat 'em ups are an ideal genre for cheap downloadable games. I love beat 'em ups but they are very one-dimensional. But I don't really mind that. I like just joining up with my brothers (co-op is MANDATORY) and beating the **** out of videogame characters for a fun half-hour or so. They work best as short games with lots of replay value. Modern attempts to pad the length of these games just doesn't work. They're like a punk rock song - tons of energy and over before you know what happened.
It isn't the type of game that you can really expect the market to pay $50 for. But a couple of bucks for a downloadable title is totally reasonable. And they wouldn't need a high dev cost either. Fans would probably prefer something more low-tech with sprites. The most ambitious beat 'em ups were titles like the Capcom's Dungeons & Dragons games. Well how much these days would such a game cost to make? The development cost would probably be low and the games themselves would be very one-dimensional. This is the perfect $2-5 game.