I know there's a lot of work to be done (and especially focus should be towards new stuff, not improving already improved stuff), but can't resist
1) Chromatic demon fight. Very well designed, I love him and the way to fight him. Just his followers that I find somewhat "generic" (although I know that it is much easier to re-use working recipees instead of creating new stuff with computers in general). This fight would be extremly cute in my opinion if demon wasn't changed but his followers were replaced by 4 liches (maybe with names...like the actual mages that lived in the laboratories) each one specialized in an element (fire, cold, electricity, acid instead of poison). Liches that would still fight
cleverly in terms of scripting, but that will have a huge preference for said element in its spellbook (and could maybe have a special custom spell based on that element for more flavour).
I think the party can handle this at that point relatively well.
2) Mind Flayers. Somoene I think it was nicoper said at some point that mind flayers at the end of sendai's enclave are not strong enough. I think mind flayers in general lack a little power...but I understand it can be hard to balance them as intelligence drain is kind of harsh as an effect. So I think an idea could be to actually change the amount of intelligence drained, depending maybe on the "level" of the mind flayer (mind flayer, ulitharid, aloon, etc))
for example, a mind flayer would drain one point per hit. This nerf would have the benefit of allowing some major improvments with mind flayers, making them a lot tougher/cunning etc, so that the intelligence drain will be manageable in a similar way that you can manage to avoid getting too many strenght drain hits in fights against other creatures.
Just two ideas, because discussing this mod is fun anyway