The only, and I mean the only, way I would be cool with always online on a single player game is if it meant we got a world that was was absolutely enormous... one that was evolving and growing... because the world itself was stored on servers, like an MMO... but the game should not be an MMO, at least not in the traditional sense... You should be able to limit the people in that world... say a max of ten, so you can play with your friends, like if you were playing the tabletop game.... But the world would have to be truly immense... Fill that world with enterable buildings, hundreds of different kinds of vehicles, diverse terrain and weather conditions, and fleshed out NPC's. In other words, an MMO world, but one where the population is mostly NPC, as opposed to a game like WoW or Everquest or whatever, where there are thousands of assholes running around, laging up the game, begging to group with you, or just being douchebags. Limit it to yourself AND, if you choose, up to 9 other people and thats it.
In that world in addition to the set goals and storylines, people should be able to create their own "events" pick a location, pick a goal, pick the number of friendlies and enemies, random rewards based off diccidulty... off you go...
You should be able to disallow objects if you think they are game breaking or harmful to immersion.... "Ie clicking a filter to disallow rpg's, so you don't have the bazooka asshole running around all the time."
The disk itself would mostly contain the character generator, which would be more indepth than anything seen before...
And the game would have to not only be multi-platform, but cross platform as well... allowing people with Playstations to play with people with X-Box's to play with people on PC's . (I never played it, but I know the Final Fantasy Online game was cross platform, at least for PS2 and PC users...)
But anything less than all of those things, would be a complete deal breaker for me.