Felltower continues to be many things.
A source of enjoyment. We are still playing Felltower half a decade after the initial delves into its lightless depths. That's semi-High Gygaxian for "five years after the first session featuring the dungeon." The fun hasn't worn off yet.
It's a source of material for GURPS books and articles. Rock trolls, octopus blossoms, some magic items I hope you'll get to see someday, rules tweaks - a lot comes out of continuous play, which a long tradition going back to the origin of tabletop role-playing games.
It's a fun outlet for our creativity. I say "our" because much of what comes out of it is a mix of things I think are fun and comments and requests from my players. Then it mixes together with game-session decisions, which sparks even more creativity.
It's a source of posts. Felltower pretty much drives this blog. That wasn't the intention of either the blog or Felltower. But it's a strong core of what keeps people reading this stuff.
So it's done a lot for me, and we've done a lot with it.
But there are things I meant to do I still haven't done.
Henchman Brigade. I still haven't run my "all 62 and 125 point delvers" session. The idea is everyone gets 3-4 PCs each. We'd run them through, enjoy them getting chopped up by monsters the 250-500 point PCs cut through like nothing, and keep the survivors as NPCs.
Nowadays this is called a "funnel" thanks to the DCC RPG, but it's an older concept. 0-level guys building into full PCs showed up in N4 Treasure Hunt by Aaron Allston, and Yaquinto's Pirates & Plunder had three tiers of pirates (extras, secondary, and main characters) and we played it with the expectation you'd keep the survivors and bury the rest where they fell.
In any case, I haven't done this yet. Right now I don't think I can - the orcs are too organized, and motly weak delvers would just get chopped up. If the orcs let them pass, word would get back the PCs that the toll had dropped, and acrimony would ensure. I may still do this with the Keep on the Borderlands, though.
Dungeon Fantasy Gamma. Now that our Gamma Terra sandbox is set, well, perhaps I could stake out a small area of Gamma World and gate in some wizards and dwarves and so on to it. They'd go through Felltower, nominally, or come from the same world, but it wouldn't be connected to the main game. Just an excuse to use the same backgrounds, rules, races, etc. without changes.
This one I might still do as a one- or two-shot game.
Run Felltower Online. I eventually got permission from my players to use Felltower with other groups. I had plans to try running it online.
I eventually gave this one up. It's just too much work. I don't really have a regular timeslot available to play. GURPS has a front-loaded time investment in chargen, which is fine for a longer game but not for a busy GM running a short game. I'd need to master Roll20 or Fantasy Grounds, and then take my made-for-tabletop maps and make versions I could show online. That's besides finding players, teaching the rules, making available things we use in play that aren't - but I hope will be - official GURPS products, the occasional playtest elements we include thanks to my freelancer work, etc. It comes with a cost beyond the expected reward.
It's also a big logistical nightmare, as fun as it might be. I don't like prep work. Plus I'm not a huge fan of GMing for people I don't know well - that's bit me multiple times. So this one isn't going to happen.
Beyond that, the game is quite good. It's worked out better than I planned, and has stayed within the hack-and-slash bounds far beyond what people tell me is possible. It's just there are things to do with it I haven't done yet.