When I first started playing ultimate, forces were simple: backhand, forehand and force middle.
Since then I have seen more complexity - specific defences to pressure certain options for the offence, such as straight-up into one-way forces to pressure hucks in general, and funnel (one-way force with straight up on one sideline in the far half) to pressure hucks down the trapped sideline.
In one game at Nationals recently, my team had big successes taking away an opposing team's down the sideline hucks, by forcing middle. But we also wanted to pressure the hucks in the middle so we went more straight up the closer the disc was to the middle of the field. Call it Mesa - flat in the middle and angled on the sides. As we were defending closer to our own endzone this transitioned into a one-way force (the huck threat disappears and having a one-way force to rely is more valuable for a guarder.
I can also see the converse being an option: straight-up in the middle, moving to straight-up in the middle, for teams that you want to see trapped on the sideline. Call it Valley (the converse of Mesa).
Have you used these forces, or anything more sophisticated than just switching a force when the disc moves more than X metres down the field (yet worth the added complexity)?
Where are we headed? Well, we have a long way to go before we reach 100+ year old sports like basketball. Check out just some of the defences you can use against the variety of screens (aka picks) the offence can try.
But as we see teams building deep rosters, engaging coaches, training more and investing time on the details of their defences, more and more sophisticated forces should arise.