Honestly, it doesn't seem like it will be too different than what the existing series 1 and series 2 could handle. It's faster, but CPU speed was never really an issue. The huge leap from S0 to S1/2 was mostly due to Apple adding an entire second core. The increased CPU and GPU power will make some things like transitions a bit smoother, but they aren't going to offer anything to developers except for some extra headroom.
I've only dabbled in WatchOS programming, but it seems that power usage is the biggest obstacle for developers. These watches have tiny batteries, and using any amount of CPU power is going to drain that battery quick. If something wasn't possible on previous watches, it's probably still not possible. That being said though, I don't know if something like interval training wasn't possible, or just that nobody has bothered to develop something yet.