I'm pretty sure the intended way to handle stairs is to simply put a 2d set of stairs on the 2nd floor, letting players know where the stairs are. There's no real benefit to having an actual hole in the floor. But, for whatever reason, I haven't adopted doing this. What I find myself doing is leaving a 3x3 area open.
What I end up with then is the desire to fill in most of that area, or have a railing to block off the exposed area. So, I messed around a little with a 3x3 tile, and here's what I came up with:
What you're looking at is a 2x3 tile that can fill in the area, leaving a 1x3 area exposed for the stairs below. I can imagine a kind of stairwell setup where you alternate these tiles, exposing the stairs on one side, then the other.
I think I accomplished one of my goals, which was to avoid having to make a lot of really custom pieces. Here's an exploded view of the pieces involved:
So, I started with a 3x3 piece, and cut 1" off one side. I then cut out two of the tab slots half their normal width. I started with the assumption that having the whole corner of the piece cut out wouldn't be structurally sound, so I figured this was a good alternative. I just chopped a corner off a normal tab. This can be inserted up-side down to go in either side of the tile.
I haven't actually kitbashed the texture yet. I just took a 3x3 tile and started cutting by hand. I will probably rework the angles of the flaps to try to get the most strength out of it as I can. If possible, I might even be able to get a full tab slot on the side that currently has none. This might be good for adding a railing to the side. It's unlikely that it's necessary to have anchors on the tile, so when I make a real texture for it, I'll probably go anchorless.
Here's another picture with the tabs lined up and ready to go in.