I'm sure Volition will do as much as they can. And I understand if they can't make a beautiful looking game AND add so much content. Even with next-gen. Most next gen games I played so far were pretending to be big open world, but they're not. They are just really slow paced. Perhaps GTA V. That game had a truely big city with loads of veriety. But then again, San Andreas was also a huge city. So either Volition will make a beautiful new or adjusted engine that allows a large, living city and tons of content that will keep you busy for +/- 70 to 80 hours and has a high replayabillity rating. Or it will (probably) go like one of the following 2 options:
1. You get a beautiful game, but a smaller city. They do what Watch_Dogs did and let you do most things on foot so the city feels big, but you have half of the activities you had in the other SR games, less costumisation, less or no choises, but hopefully a long story that is fun to play through and still keeps you busy for about 20 to 30 hours if you want to complete everything.
2. They make a less beautiful game, bit outdated graphics, but a huge, living city a la Stillwater with lots of content, more activities, racing, costumisation options, clothes, player choises (Although they probably should try to make one ending you can influence, but the outcome is still generally the same for the next SR game. Don't make the choise between "save Shaundi and let Pierce die or the other way around" while in the next SR both are alive again.) but you can influance the ending still. MAke it more easy because of things you did, or harder, have different ways to achieve that ending because of choises, that sort of things. BUT because of ALL that content, the game won't LOOK as a next gen game, but more like a more pretty version of SR:IV.
Personally I hope for option 2 (if they can't combine both of course.) I'd rather have a SR game that still looks nice enough but has a lot of fun to be had in the game itself. SR2 was also outdated, and still one of the most fun games I ever played. Everthing in that game felt great (except the scene where ultor soldiers raided a bar and the boss pulled the waitress onto the bar as a human shield. That broke immersion for me. I tried to play the game semi-good as much as the game allowed it. Shooting cops in the knees and such. xD )
But yeah, that is my personal opinion. I just wish they gave more information about it.