And FWIW, I'm not sure I totally agree that a rotor is the only thing that would make sense, if the idea is that a valve of an appropriate bore could be constructed.
The big reason why I say that is because alto trombones are not generally used for low notes. Large bore tenors certainly are, so having a nice open valve that facilitates an easy low register makes sense. But a good rotor on an alto is more than enough for what you would need it for on alto, and it takes up the least space on the gooseneck. Of course, if you're carving out a new niche for the alto trombone or wanting to make an extra-large Eb tenor (in the same way that we have an extra-large Bb bass), then a small Thayer might make some sense.
I'm just a touch surprised that altos in late 80s and throughout the 90s didn't follow the trend of large bore tenors at least to some degree.
I think the only reason why is because alto was rarely used at all back then. If alto had never fallen out of favor in the first place and remained a standard part of the orchestra, I'm sure we would have had weaponized large-bore altos with Thayers and big bells during that time.
A 36CL with a 500/508 and a .530 rotor or something seems like it would blend really well with a larger tenor/bass.
In my experience, a .547" alto with a valve designed from the ground up to match a modern orchestral tenor/bass is what really works and can keep up and blend at any dynamic. I think .500/.508" would still be much too small honestly. If altos were dragged into the '80s/'90s arms race, we'd probably just have a lot more .547" altos today than the one kind that made it to production (Pfretzschner).
Anyway, that's way too many controversial logs on the fire. I don't want to start dousing it in gas.
Keep it coming, baby!