I'm finally going through the SP campaign of GTA5 on XBox One (after I originally did it on PC a couple of years ago). So many things are missing from GTA Online. They're little things, but I definitely miss them. I wish for a local multiplayer mode that merges all GTA 5 features (except the campaign itself) into GTA Online features, but without official recognition by GTA servers, of course. It's too easy to cheat if unmonitored, and money (which can have real-world value in GTA Online) is too much of a risk in that scenario.
What do I miss from GTA 5? Critters galore on land and sea and air. Random criminals. Police patrols and random car chases. Active movie sets. Active airports. Being able to see activity miles away. Visiting the comedy nightclub. Strangers and Freaks. Individual store inventories. ... and more. These things could only be synchronized over local MP on small services and not the official massive services across the Internet. ... and one could add even more features on unofficial self-hosted services like custom heists or game modes or general content.
Essentially, I want to share my GTA 5 experience with people. GTA Online goes about 60% of the way there and, then, veers off to far-off destinations.
(Also, GTA 5 is still gorgeous on PC. Too bad PCs are so easy to hack clients. GTA Online, while still prettier on PC, still only goes about 60% of GTA 5.)