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.