I sure hope that LC will be added. We may be able to rob the Bank of Liberty when heists come out.
However, some map changes will be required to prevent plot holes + make it suitable for GTA Online.
-Construction sites finished (and maybe new construction)
-Maybe even the new Freedom Tower could be implemented. It is now the tallest building in NYC, and in TBOGT Yusuf said he was going to have the Amir tower be the tallest building in the city.
-An underwater sea bed portion would have to be added.
-They would want to consider using the PC trees, unless the season is different.
-Seasonal change. This one depends. GTA V is set during Winter as North Yankton is snowy. However GTA Online is set before GTA V, so it could be any season. However if its Spring, Summer, or Fall, and they add North Yankton, the season would have to match. That or they could even make the season change depending on the time of year it is when your playing. There are a few games I know of that do this.
-Add Ammunation. Why? Because in GTA Chinatown Wars (set in 2009), you can order weapons from Ammunation on the ingame internet, so that means the gun laws are no longer present in LC.
-Add the LC Gun Club. It was there in Chinatown Wars. This could be where you do target practice like at Amunation. It could even have an obstacle course like in CTW (and Vice City), in addition to the standard shooting range.
-Add locations for Apartments, and maybe even houses and mansions (which should be done in LS too).
-Change bill boards to match GTA Online's time.
-What would be done with Pay N Sprays? Maybe keep them the same, but they would work like LS Customs?
-Add interiors for convenience stores so they can be robbed.
-Determine what is in place of the buildings that were destroyed during the events of GTA IV, TLAD and TBOGT.
Other things that would need to be worked on.
-LC radio stations. You can't just listen to the LS radio stations. If they were lazy, they could have the GTA IV DJ's, and the stations would play the music from the corresponding genre. (Los Santos Rock Radio:Liberty Rock Radio, Radio Los Santos:The Beat 102.7, Channel X:LCHC, West Cost Classics:The Classics, etc) However I say they add songs that play on both the LS and LC radios. They can't add back the IV/EFLC songs as they would have to re licence all of them. I'd say adding new songs, and the stations of the same genre in LS and LC play the same music.
-LC Peds. They could reuse the GTA IV peds if the season is in the fall, or they can reuse the LS peds if its in the Summer. However, all the peds who are on the job would need to be changed (cops, firemen, paramedics, etc)
-Add vehicles. (Emergency, Taxi, etc... They could be GTA V models but painted to be used in LC, as the V models fit with the time of GTA Online.)
-Add LC plates.
-Games added in LC for all game modes.
-Missions added in LC.
Optional things to enhance it.
-Airport interior in both LC and LS.
-Spawn 747's at LC and LS airports.
-Working cargo plane, where you can put your car in the back.
-Flights between the two cities.