I just had this idea last night and I spent quite a lot of time thinking about it,
Would it be possible to ever have a completely new Garry's Mod? Let's say, something that uses a totally new physics and graphics engine, has the possibility for HUGE maps and has integrated coding for all the stuff we consider to be troublesome with the current game engine, such as perfectly solid constraints, bug-free parenting, no speed limit, functional high RPM's for wheels any many many more. Better graphics combined with exponentially bigger maps (or even worlds) would open a whole new field for fast vehicles and RP-possibilities.
The physics engine would not only emulate mechanics, but maybe some more niche-aspects like electricity, thermodynamics and aerodynamics? Who knows.
The problem I see is the huge amount of work required to create something like this, and the funding part comes in play as well — It would be a risk to take up a project like this and expect it to be popular. We have Minecraft; it's not very much comparable to GMod, I know that, but it has shown the potential popularity that sandbox-type games can achieve. They just need to be accessible enough for new players to enjoy it, and deep enough to allow more advanced players to build complex creations.
So, in addition to the question in the title, what would you personally love to see in a theoretical, new GMod?