I've got to the point with my mod where I want to add an extra City Gate to Leyawiin to lead to the (new) docks and this will obviously create an incompatibility with Open Cities. What are my options?

Do I create two versions - OC-compatible and non-OC? One version plus a patch? Right now I don't have OC installed, as I'm mod-building on an essentially vanilla OB/SI, adding only the UOP's and OBSE, but I may have to do so soon - is it easy to turn on/off so I can test both ways?

I won't go over the details, but I illustrate one way of accomplishing a switch for Open Cities within one of my old mods
http://www.tesnexus.com/downloads/file.php?id=6084
(adjusts for skingrad and chorrol)

Basically, I use a few scripts to detect which version of the space the player or NPCs are moving through, and make an assumption based on those checks which enables/disables markers, or groups of statics to adjust to this. You can do it more directly with OBSE by checking to see if an Open Cities mod is loaded, but using known and persistent NPCs (such as beggars and others who wander about) tends to be a bit more reliable as it catches all instances of the city being open.

It would appear that I'll need two esps whichever way I do this, as the new South Gate will need to use the modified CastleGate01 from the Open Cities esm, and its inclusion, even if disabled, would require that to be present. So now it's a question of two versions, or base plus patch. I'm guessing that I'll actually have to build the latter way, even if I merge the patch into the base to create an OC version for easier download and fewest esp's needed.

Right now I'm happy with the methods for detecting that a city has been opened, and I can add code to some scripts to adjust so that they use the replacements where applicable. Right now, it appears that the Lucky Old Lady statue is the only place I have any work to do in that respect.

However, I haven't found any discussion of how to edit the cells containing the walls. In particular, the Leyawiin South gate that would need to be conventional for non-OC users, and opening to match the others for OC compatibility. I'll need to create a version of the cell for OC as well as the vanilla ones (inside and out), and make sure the user replaces the right one when they install the mod. The OC version has to have all the content inside the gate, which has to match my modified inside and outside cells as well as being consistent with the adjacent OC cells, and anything that adds (the AI gate is one corner of it!)

Has anyone other than Arthmoor done this before? It looks like all the patches come with OC itself.

Enabling/disabling of objects based on the state I mention above should work.

I don't think you can do that with entire cells.

The cell containing a gate is two separate cells in the vanilla game, one outside in the Tamriel worldspace, and one inside in the city worldspace, with the detail meshes like trees, rocks, and clutter only on one side of the wall. The open version is a single cell combining the original two and containing all the stuff from both and part of the Tamriel worldspace. The inner cells of the city that don't span the walls also are cloned into the Tamriel worldspace, which is why the Lucky Old Lady is affected even in the middle of Bravil, away from the walls.

Exactly. The Open Cities version of both sides of that gate is put in place where it should be. The non-Open Cities version is put i the same place it should be, meaning that you have some overlap. To get around this overlap you figure out which version the statics belong to, parent them to an activator, and disable/enable that activator based on the OC state. The inside of a city still exists within vanilla, it just isn't normally reachable.

The only thing that you can't change with this method is landscape, but given how you're adding an exterior gate that is present regardless of version, that shouldn't be an issue.