I was sick of walking all the way through Riften to get to the Thieves Guild to trade with Tonilia, so I made this really simple and crude mod to have a shortcut between Honeyside and The Ragged Flagon. It's got a navmesh in it too, so your companions should be able to go through it, as long as they're human sized or smaller.

It was a while since I made this, so I forget exactly what I did for features, but I wanted to upload it here in case anyone cared about it.
It should only appear when you upgrade the house to include the Housecarl's Quarters. I replaced the rug in that room with a "hidden hatch", and in the Ragged Flagon on the other end is a little hatch in the gutter that appears once the room is enabled too. I don't know what this does for the continuity of the Thieves Guild quest, IE skipping killing those guys in the Ratway, but I don't think it really matters much, if there are any issues, I suppose you can feel free to report them to me.

Incidentally, I have no idea how conflicts work in Skyrim, but I've always loaded it early in my load order just in case anything wanted to take precedence over it and I've never had any issues yet. I don't know if this would have any compatibility issues, again, feel free to report those, but unfortunately I might be too lazy to fix them or not know how to at all.

I wanted to make it so that you could buy the tunnel upgrade from Delvin Mallory but i couldn't figure out how to do it, if anyone has any insights I might try again some time

So about the user/editing/distribution permissions and stuff, I don't really care about these permission things, if anyone REALLY wants to do that stuff with this, I guess just ask me and I'll enable the things as necessary. This is kind of just a personal mod, so I don't see how any of that would be important.