An API to allow add-ons to save and load their own gate properties to/from SG's database.
Possibly a new table?

This might be the harder option as it modifies existing tables, but we could add a data column in the Portal and PortalPosition table. So that would be a text type column, where other data is stored like a json string. This would behave very similarly to minecrafts metadata storage

It might also be a good idea to add the ability to add flags using the API, and to add a portalpositiontype. Don't know how necessary it is to remove stuff in this case, but if it's easy to implement, why not

So probably run statements like this:

ALTER TABLE PortalPosition 
    ADD data TEXT;

And

ALTER TABLE Portal
    ADD data TEXT;

And have them inside a try/catch statement. This data should also be retrievable, and modifiable. Also does not matter if it's null.

Note that this has to take interserver databases into consideration as well

Format of those text should be like:

{DyeColor:"WHITE", AnimationsFile:"/plugins/StargateExtraColors/animations/animation1", textColor:"#000000", higlightColor:"#FFFFFF"}

Not tested, but implemented