There's one feature I've been searching for and have yet to find in any other file manager or even as a request here and that's adding the ability have Solid (optionally of course) physically serve as a replacement for the stock documentsUI found in Marshmallow and Nougat.

I've got several reasons for thinking this is not only a good idea but beneficial for the practicality of the all-in-one model Solid seems to be striving toward, and those reasons are as follows

1 - Everyone loves removing bloatware, and thats pretty much what the documents provider is when sitting next to something like Solid,
Seems logical to ditch the little file handler with all of 3 functions for the powerhouse sitting a few folders down. So less overall impact on consumed space.

2 - If you've ever tried to show an app a directory that was outside its reach, like perhaps a music folder on an OTG drive that the app just didnt have the permissions for, but it did have to ability to attach it's self to the stock Content Provider and be given access to a directory, then you'll understand why giving Solid that privilege could be super powerful given its extraordinary reach across ALL Storage Points, and isnt limited to the internal and OTG storage only like DocumentsUI is. (Apps like Titanium Backup could then leverage that and be able to see and access LAN/SMB directories) and finally provide a practical solution for devices with no SDcard slots, to use backup utility tools that would outerwise use precious ever shrinking internal storage. (if you could use them at all) I know Nexus users and Pixel particularly would flip over such a thing, my own device also the Oneplus3 and I;m sure many more would benefit. Seeing manufactures all opting to not provide sd slots more and more, means we've got to start thinking outside the box to maintain our ever shrinking space, this seems like a good place to start.

For other reasons as well, but my post got way big way fast so I'll try to wrap it up...

I looked into what it would take for this to happen, and best I can tell the API for building a System Content Provider is available, and correct me if I'm wrong, but Solid seems to already meet most of the prerequisites required to handle the job,
Addin some new class calls, add some perms to the manifest, and some lite UI additions for things like image calls with a side panel apps list to hand off to, etc

I;m obviously not a programmer, nor do I assume to know what it would really take to pull off such a thing, but damn wouldn't it be nice!?