It would be nice to be able to launch other projects on the same gateway in a new Designer application instance from within an already-open Designer, instead of needing to go back to the Designer launcher. This would save a little time.
E.g.
File-> Open in New Instance
I fully agree with this!
I'd add, and I know this would be a much more involved ask, but it would be great to be able to have two (or more) projects open within the same Designer instance. Something like a project tab at the top or bottom. That way you could easily tab between a project and its related global project. When you have designers open from multiple servers at the same time for different projects, plus a bunch of other applications, it gets painful trying to ping pong back and forth from one project to the correct, related, global project Designer instance. Having it self contained within the same Designer instance would be a huge help on this front. The ability to auto open related global projects when you launch a Designer instance would be icing on the cake 
That would be super useful as well, although I do also like to have multiple Designers open at once as I can then have them open on other monitors. But I can definitely also see the benefit of having the ability to quickly edit something in another project within the same instance. Probably a tonne more work though, as you say 
I'll say that 'Open in New Instance' is infinitely more likely to happen than 'Open Multiple Projects'.
That would legitimately be an undertaking on par with just rewriting the entire designer.
Haha, that would have been my guess as well. Nothing you couldn't handle over a weekend though, eh? 
Completely agree, and I wouldn't suggest it as a replacement for having the ability to launch projects in separate designer instances. Ideally you would have the option in the tabbed designer to do something like a "right click >> launch in new window" too.
Ha! I figured, but I had to throw it out there. To me this was the biggest loss from the old global resources system. Not an insignificant amount of time lost searching through open applications to find the correct one...probably means I just need to close some applications!