Not an expert by any means but I'm really curious about this as well, and I've done a lot of poking around and experimenting. I'm especially curious about whatever Gateron uses in their Ink housings...
Unless I'm confused, Holy Panda (Halo) stems are made out of the same POM blend used for most other stems. UHMWPE is a relatively recent addition to the MX-compatible arena, I believe - I've seen a few posters talking about the ones they have as experiments and/or prototypes.
I think there is still plenty of room for innovation in this space - I think the exciting stuff has just begun in terms of how many people are becoming curious about this stuff and experimenting with it. It's like crowd-sourced development.
That's how we got Holy Pandas - which in turn brought new folks into the hobby and inspired a whole load of other really good switches; Zeal V2s, T1s, BOX Royals... even vendors & manufacturers are embracing the framework of kitbashing with the Hako Royal switches - a mashup between Input Club's Hakos and NovelKeys' Royals. I've got some Tangies, Creams, and Inks all on the way purely because my brain wants to know the optimal configuration of plastics for a satisfyingly smooth and aurally pleasant switch. I really could have stopped by now, having found loads of great switches... but they just keep getting better!
Beyond fiddling with endless variations on the same design, there's also room for innovation with new designs - take the BOX Silents I'm typing on now. On one hand, it's easy to see how they are derivative of MX switches, but it's also easy to see what makes them different once you pop one open. (Enclosed contacts, how the stem is cradled in there)... I think the BOX switches represent the longest single developmental stride the MX-compatible industry has made in a very long time - but there's still plenty to improve on.
Personally, I think the next frontier beyond smoothness is stability - and I think we've gone about as far as we can go with the traditional MX design from the 70's as far as that's concerned. There are those backlight-centered switches sold as Romer-G's - they're innoavative in their own way, though I also think they introduce new problems - but hey, that's the point. Experiment!
