MNC stands for modified number of cities, I'll explain in a second. People usually say "there was Liberty city, then Vice city, then this, then that, so that must mean that we will have Vice city or Liberty city/London,... now again." Then someone says "no, that's already been done, we will have something totally new"
However..what if they are both wrong and correct at the same time?
Let's forget about anything before SA for a moment. We had SA, after which we had LC, but then we got SA again. Exceeept, we got a SA with a modified number of cities, so instead of LS, LV and SF, we got just LS.
What if Liberty city is next, but with a modified number of cities, this time going in the opposite direction, adding new cities along with LC?
So instead of just LC, we'd also get probably Boston, Washington, Philadelphia and Baltimore. They are all more or less well known places, big cities, have suburbs, have industry parts, ports, have rail/highway network between them, airports and all other goodies.
As large as SA with just LS is, it feels pretty empty once you're outside of the city. You have trains, but you don't have anywhere to really go with them, except 2, 3 villages/ small towns. You've got a huge airport and again, nowhere to really go with it (except 2 runways in the desert)
By adding a additional 3, 4 or at least 2 cities, public transport services would actually serve a purpose and would be much more enjoyable. Maybe more importantly, highways would also serve a purpose, currently you just go to the north part of the map, spend a few minutes there, get bored and return to LS, but if you had multiple cities, you could spend hours in one city, then travel to another one and so on and traveling to a new city would feel like a actual adventure, like when you discovered a new city in GTA SA for the first time. Connecting stories and characters between all those cities, there's just endless options.
If instead of this, we got another version of just LC, VC or something, we would have the same thing happen like now, 1/3 or 1/2 of the map which is fun and the rest of it which is just a desert/forest with a few smaller villages.