It would probably end up destroying the franchise for me, because I don't care about GTA's online aspects at all, and have a strict policy of never playing MMOs after trying WoW for a few days, as well as a couple of others (Everquest, and some other popular one I forget). To me, they just feel like those phone games where they want to reel you in for more and more grinding and selling you extra garbage.

I'm old school, from the time when paying for online was simply unthinkable. And MMOs are usually the exact opposite of that. In my case, I LOVED Warcraft 2 and 3, and rather liked WC1, though admittedly WC1 was before Blizzard had achieved their famous levels of polish and pedigree. WoW basically destroyed WC for me, because instead of getting WC4, we simply got more and more add-ons of a money sink that to me was just an endless collectathon. I'm not going to pay monthly for some treadmill, I just don't enjoy that kind of thing personally. If both things existed, WoW alongside WC4, WC5, etc, then I would have been absolutely fine with it, as everyone is happy. But they wanted to keep people in their money-mill, and the original fans of the franchise were simply SOL if they wanted more of the RTS gameplay.

Anyway, as long as they left the GTA series alone, and kept making one every 5 or so years as a full scale mainline game, I'd be totally fine with having a GTA MMO out there for MMO fans to enjoy. However, I fear that they would simply abandon the expensive, comprehensive world-building of the regular games, and GTA MMO would be the ONLY GTA that they offered, in order to maximize their profits of the monthly fees.

I would prefer it if they still focused on the single player aspect of it. Obviously, the online component has been extremely successful for them and it's nice to see they have supported it really well. However, I really think the stories they tell through single-player are generally well done.

Edit*

I misread the first part of your post.  An MMO mode could be interesting as long as it doesn't hinder the single-player mode.  Rockstar have proven they have the chops to make an engaging online experience but and MMO is a different beast.

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Well I'd like them to make a brand new gta. For example gta 6 back in vice city. Awesome big world and single player as usual.

But then the online. Make it vice city online but also keep gta V as a second map. so you basically double the size of gta online.