I think the heist theme was certainly an interesting idea but they didn't know how to use it well. You can't have a whole game revolve around that, it would fit in a movie but not in a long game like V, then we already have Michael being rich as hell and even if he wasn't how many heists would they have to pull for themselves to have enough money? Not that many, unless they keep f*cking up somehow.
So they invented the whole FIB angle and had you do most of the heists for them which was awful and the definition of blue balls. The protags feel like bitches of the FIB and barely get any money..
They should have gone with focus on real criminal organizations again and just have 2-3 heists in the game, that would be better. Apart from the first heist, there wasn't one that came close to GTA IV heist for me, they just felt more like over the top missions.
I agree that the FIB was misused and the IAA shouldn't have even been in the game aside from a little cameo. But they could have easily done a game purely focused on thievery: they start off small (the jewelry heist was too much to start off with) and gradually work their way up to bigger heists, the missions in between could revolve around them building a criminal network of money launderers, hackers, lawyers, people to run their business fronts etc. who will also take a cut from your heists so you're not making too much money. More properties to purchase, maybe have a few tussles with the gangs but later on be able to hire some as muscle or as diversions during police chases and such. And just have the story be a classic cops and robbers sort of thing with the LSPD and the FIB as the primary antagonists.
I could go on forever with this fantasy version of GTA V I've made up in my head, but I'll just cite a recent open world game the did gangs well and way better than GTA V but was not a gang focused game: Love it or hate it, Watchdogs 2 had gangs, they made their presence felt within the game world, but they never served as a vital part of the story aside from being included in a few story missions as secondary antagonists and as enemies or obstacles for side activities. You interact with them waaay more than you do with the gangs in V, and this is game about hipster hackers sticking it to corporations.