It is impossible to say, and I have been very worng on these topics in the past, probably because I'm less aware of how popular bands I dislike are, but I'll make an attempt here.
1. The "orchestral" side of metal is evolving. I can't say where to, but it seems to be getting higher into the orchestral sophistication. The new kamelot album has a lot of that, as does epica's consign to oblivion. It possibly is peaking though, since the popularity of orchestral metal is bound to decline, and less money = less orchestralism.
2. Post metal is a genre to watch. It keeps evolving, because all the bands seem to be very in tune with each other, few innovations go unnoticed.
3. Math metal is the next direction in technicality. Math hardcore thankfully is nying out, but prog metal has been eying it, and I assume death metal bands will at least begin to leave 4/4 in search of unique riffs, although less will explore polymeters like meshuggah since it is so hard to do real well that only the best can do it. Of course, that won't keep idiot fans from calling anything complex a polymeter. If the african scene picks up, however, then you can bet there will be a lot of polymeter.
4. rapider riff shifts. On none so vile by cryptopsy, it is heavy as crap because they have one riff every 2-3 seconds on some parts, often repeating a collection of riffs instead of repeating a riff. Dillinger escape plan did a lot of this on calculating infinity. I think it is possible to take this farther, and move past riff-based metal into a more atmospheric metal where the progression of riffs is more important then the actual riffs. In be-bop jazz, there is a similar thing. The notes are really fast, and melody is less important then before.
5. electronics. Keyboards will never make it, but there is a whole world of effects out there, and I think that more and more strange effect pedals will be used. Eventually one will sound so cool that everyone starts using it, and the obsession with this effect will lead to music being created around it, and the next big genre will emerge.
(1-4 are predictions, 5 is just speculation into one possible future)