I'm a bit intrigued with GTA not being set in a big well-crowded city. A series of small to medium sized towns could be a great change of pace. But I felt that GTA should always be set on a large-scale metropolis riddled with vice, crime, corruption, and greed. GTA is all about stories focused on organized crime, with elements of satire, self deprecating humor, and player freedom. I feel that a big satirical rendition of a US-based metropolis is the way to go for GTA to succeed. GTA is also all about homage to some of cinemas greatest hits, especially within the crime genre. Movies like Scarface, Heat, Pulp Fiction, Reservoir Dogs, Goodfellas, etc. are the biggest inspiration/s of GTA both as a cultural phenomenon and as tribute to some of the most brilliant films of all time. Because at the end of the day, the guys at Rockstar show care and love for cinema, they showcase their talents to give us immersive worlds inspired by these movies, and they show their passion to do what they love.
So for me, it is necessary for GTA to have cultural impact for their audience, and for entertainment's sake, I felt that only a big city can cherish what GTA stands for, and to exhibit witty, and complex storytelling that GTA can only do. Lastly, I of course will be fine if there are small towns across the map, but it would feel shallow and incomplete without a metropolis to compliment these small towns.