I think I found what's next. Everywhere I go, people are talking about those mud runs/military runs/”Tough Mudder” runs. I heard it in the locker room after my run today (discussed by people coming out of a group fitness class, not by people coming in from a run), I just heard some people walking by my desk talking about it, I've had a few people I know ask me if I would be participating in one. While I hold absolutely zero interest in these events, they seem to be rapidly gaining popularity. If you can call those running events, as many people seem to want to call them, they are the next big thing in running events.
Good point, I have noticed those (either as stand-alone events or a nationwide series of races) springing up quickly all around here, too. Like you, neither I nor any runner I know (including a Scottish Fell Running champ from days of yore and several competitive trail runners) would ever bother with them.