Well, expanding the creative element is what I'd like to see for example, be it a course creator tool which offers more possibilities to create a truly unique track (including the scenery), much like the ModNationRacers one, or extending that aspect towards the cars.
Perhaps a full car design tool is far too complex but maybe more aimed towards designing aftermarket parts like bodykits/spoilers (or change the lights, etc.) or being able to alter existing ones, finetuning them to your personal taste.
I know that'll sound to many as NFS and not entirely new but maybe a more subtle approach within certain limitations could work within the context of GT.
Maybe a logo-editor where you can design your own logos and apply it to your creation and the ability to change the cars name (only possible when a car is modified/customized), I think many tuners in the tuning section would love to offer their cars with their own name and logo, much like Brabus or Alpina (although licensing may prevent that, perhaps an additional name might be possible).
How about an engineering mode where you could design complete engines or have to figure out yourself how to stiffen or alter an existing chassis instead of the default option in GT-Auto?
As for a livery editor, why not extend that to suits and helmets as well, no need to browse through an endless supply of different coloured gear when you can create your own (based on a few default options) with your own name on it or whatever.
Is that enough for now?
To be honest, some of those ideas might be trivial and right now I'll just settle for a livery editor, as that feature alone would take
a lot of my time.
Who cares if it's already in Forza? So are cars and tracks.