The main improvement to SSGI that should take priority is to make it radius/thickness aware.
Currently the biggest issues with SSGI is it’s unusable if your project has foliage, in particular grass.
The main issue is you want SSGI to give you nice ambient shadowing, which makes grass unbelievably, and unrealistically dark, and shimmering as well.
Lowering the parameter for AO leads to a massive reduction in AO/contact shadows for all your objects, and so although the grass issue is gone, SSGI becomes pretty useless and hard to excuse for its costs, you might as well just use SSAO and disable SSGI at that point.
After that imho, another nice improvement is to make it less reliant on excessive blurry denoising. Things get pretty blurry with SSGI.
Imo, the main ideal usage for SSGI should be with APV - and it should be improved and worked at in the future with that use case in mind as a priority, it would be much more useful than just SSGI by itself. Obviously it already works with APV now, but I mean future development should see that as the biggest/most important use case.
Just my two cents 