Because it adds another area to explore, and quite frankly, it's not as boring as half your ideas.
"y wuld u want to ad another city we just left lc come on man. BUT HOWZ BOUT MORE MISSIONS IN LOS SANTOS." Sorry, but if you offer me A WHOLE OTHER MAP or just some boring ass missions, I'd easily side with the whole other map.
THAT also being said, I doubt we'll get LC. Mainly because, effort. If anything, I'd suggest a more reasonable DLC.
Odds are, due to multiple endings, we're getting one prior to the events. Or.. there's a good chance of it? I don't know. I say, go North Yankton style. Open up airports to go to North Yankton after the DLC is complete, release it on GTA:O, deliver a dark and gritty story with some humor due to Trevor about Micheal and Trevor's encounter and what they did over there. Release North Yankton on GTA:O as well. Problem solved.
Franklin fans can get DLC 2 which explains the downfall of Grove Street and what happened that made the Ballas take over.
I was always thinking of a "North Yankton Stories" DLC...
It can make all of North Yankton accessible instead of just Ludendorff. The story can start on Tracey's 13th birthday in 2004 at a time where Michael really wants out of the criminal lifestyle. Jimmy would be 11 in the game, so your children in the DLC are actually children. You live with them and Amanda just like in the main game, but instead in trailer homes. The first few missions can consist of him and Trevor preparing a heist somewhere else in North Yankton other than Ludendorff and arguing with Brad most of the time. I'm thinking Brad and Trevor should have a Brian-Billy relationship from TLaD, Brad is always siding and kissing Trevor's ass but Trevor doesn't exactly like it. Right before they go through with the first heist, Michael should meet Dave Norton and help him out with some very small time FIB work (not like in the main game, more like GTA IV's UL Paper stuff - both help out each other). Norton's career was made after taking down Townley, so the FIB work here would not be tactical or Call of Duty-esque at all, more like vigilante work from past GTA games, but obviously more in-depth and driven. Maybe even make it a Side-Job, like TBoGT's Drug Wars or TLaD's Gang Wars. Back on topic - soon the M, T and B go through with the first heist in the game and have to skip town and lay low in Ludendorff. While planning the Ludendorff heist, Micheal seals in the deal with Norton but only on one condition - Trevor has to be taken out, too. That would make Michael angry and he would try to work something else out. Norton would refuse and Michael would have to put family before friends. Right before the last mission, Michael has the conversation with Amanda that is heard in Michael's head in the mission "Bury The Hatchet". Eventually, the Prologue heist comes up and acts as the last mission in the DLC.
I would be expecting more story to go into the DLC, but what i wrote would be the perfect basis of it. I know almost for a fact it won't happen, but i hope it does. I loved the first mission's feel and it could be made even better by adding this prequel. It would show Michael's mindset while betraying his best friend, Trevor at a time where he a little more controllable and Brad being the annoying dick he was made out to be in the main game. The tone of the game would certainly be darker and grittier by the story alone. The snowy, rural state of North Yankton would even further enhance this tone. It would be too cool if Rockstar made this, and i would gladly pay $20, maybe more if DLCs can be more expensive.