@z970
Carbon 1.6.1 is an obvious one, as well as a newer Darwin and Cocoa, if possible, but those are less realistic.
If it were possible to strip the Blue Box menu bar and integrate it into the X menu bar more cleanly, that would be a very nice to have feature that could possibly be backported to Classic if it works similarly. Admittedly I don't know if that's possible, I just skimmed the idea off Frogfind.
General optimization and bugfixing is another obvious one.
Tear-away menus was a cool feature in NeXTSTEP that I use all the time in the Window Maker desktop, and I've seen people grumble about their removal from X.
If a dock is included, it'd be nice if it was optional, and the option of the Control Strip would be nice if possible and not too inconvenient for quick access to settings.
I don't
expect this, but a rough implementation of relatively modern Web capability (around Tiger's level) is a nice-to-have. And 10.13's security certificates if it's still the same format.
At least a G3 minimum would be very nice, maybe a 603e if looking for a challenge but if the web stuff bumps it up to G3, that's fine, 9.2.2 was G3 only as well IIRC.
I'm sure there are others I'm forgetting or just have never encountered. Maybe an option to either use the more NeXTish Rhapsody UI or the more Classic Mac-styled 10.0 DP2 UI? That would probably be unrealistic as you'd have to maintain two (or more if you count DR1 and DR2 as being different enough to warrant separating) different yet still integrated desktops. Generally just more hardware support? That seems like it's obvious enough not to need saying, but I'm just putting it in here to make sure I've exhausted all of what I had in mind.
A completely unrealistic fantasy scenario would be support for POWER9 and more recent hardware. Just getting G3s to be viable daily drivers to cut down on e-waste and legitimize older computers and architecture diversity as more than just toys for the sole purpose of video gaming is my dream.
Oh, almost forgot, a Startup Disk item in the Control Strip!