In my opinion the first and primary reason why we have access to so few locations pertains to the amount of officers in the game. Considering how many officers it takes to preform tasks it would be very difficult for factions that aren't led by Cao Cao, Liu Bei, or Sun Quan to garrison the territories if we doubled the amount of cities in the game. I mean sure we can add a couple more cities to the Qing province but how will Kong Rong run them with his average of 3-4 officers (including himself) Sure Kong Zhu can run another city with the one other guy in his force right?

Another barrier is late game difficulty. Let's face it, if we add a large amount of cities to the game in the end Cao Pi/Rui is going to be in charge of the majority of them. and if you are playing as him the game will be freakishly easy while if you are poor Gongsun Kang you are just screwed. Even Liu Chan, or Sun Quan would be hard pressed to face a Wei who has a dozen more cities pumping out armies, even if they had a few more of their own.
Game length is another issue, games would take far longer and Koei would need to keep things interesting enough to justify the increased length.
There are ways to bypass how empty things feel without increasing the amount of cities to a large degree. I personally felt that the map in RTK 10 felt pretty full even though it only had a few more cities then in 11. The inclusion of places where you could build camps,forts, made the game seem less empty without creating new locations to garrison troops or officers. in 9 You could put armies in gates and Forts allowing them to be actual real bases that simply didn't have the extra advantages of cities.
As for solutions, the obvious answer would be to put in more officers, but the question would be from where? Well SGZ has quite a few characters that only get named once or twice and never appear in the game. I remember reading recently Dong Zhou's bio and it listed a ton of civil officials who Dong Zhou executed that just aren't in the game. I believe Zhuge Liang's bio mentions several officers who were executed alongside Ma Su who don't appear in the games. I am sure that there are quite a few other characters who could be given sub par stats and tossed in to help fill new cities. However there is a flaw to this plan. The only groups that will benefit from this will be larger factions that have these throw away characters. Yan Baihu, or Kong Zhu aren't going to get any of these officers more then likely, so it will mainly be larger factions that don't really need more people that will get these officers.
Koei could make up new people to give to smaller factions,perhaps have an option to include some "made up daughters" that each faction leader can have join their force. This could help balance the games but it might upset fans who want a more "pure" experience that would get ruined with made up officers being sprinkled around. After all, we we really want to buff up smaller groups we can create and assign officers ourselves. However we cannot make more cities ourselves.
I really want opinions as to how we could include more cities while still keeping the game fun. Nobunaga's Amition has far far more cities and castles while having far far far far less landmass, I admit that late game Sphere of Influence can be a drag, trying to conquer so many locations when you are already the undisputed winner of the game can be very annoying. However i love looking at how many locations there are in comparison to RTK 11.
Let me know what you all think, are there solutions to this or is this something we just have to accept from a setting that is relying on records from 1800 years ago.