I remember in the 3D era, there were a lot of mods that tried to combine Liberty City and Vice City, or Liberty / Vice City and San Andreas, or even all three of them. It was amazing to see what could be done.
You are refering to GTA: United, which combines 3D-era Liberty City with Vice City - both replace the San Andreas map. The initial goal of the project was it to add the two maps to Los Santos, San Fierro and Las Vanturas without any replacement, which wasn't possible after all due to the limitations of the RenderWare engine. There was another project, which you might know, it's called GTAIV: San Andreas, which replaces Liberty City with San Andreas, but it was a technical challenge, as far as I remember. The reason for this was that the San Andreas map is about twice as big compared to HD-era LC.
I remember Rockstar stated they would like to create a game in the future, which includes every city they have ever created for the series. This sounds like an interesting idea, and I think it's feasible now that there is GTA: Online. It wouldn't surprise me if Liberty City and of course other cities, too, will be added in the foreseeable future. Modders will perhaps find an opportunity to do the same in single-player.