Both can and absolutely WILL improve their overall games, neither has completely peaked yet (though I'd say Jones is far closer to his full potential). They're both extremely young still (I'm 4 years older than both of them). That said, I completely disagree with the poster that said Jones has more raw talent. Alex imo, and in most other's that I've read, believe he has an even higher ceiling (more capacity to get better). Need proof? Jones' wrestling is about as good as it'll ever get and his kickboxing can only get so much better too. His actual boxing game has had lots of time to get noticeably better too and hasn't.
Gustafsson, on the other hand, took his TDD to another level entirely over the last couple years. He also really made his footwork a lot better and overall movement since his first few fights in the UFC. Gustafsson is still pretty raw with his knees, kicks, elbows and actual wrestling. However, his leg kicks are pretty nice and his knees certainly are coming along (he was blasting Shogun in the Thai clinch with them). He has more to improve on and utilize than Jones does in terms of his full MMA game (his entire grappling game and FULL Muay Thai striking arsenal). Jones is already taking close to full advantage of his kickboxing, he just really needs to sharpen his boxing up (a lot, yes, assuming he can). The boxing part of "KickBOXING" is the hardest part to sharpen up. Kicks, knees and elbows don't require nearly as many hours in the gym honing to get them there.
As for the rematch, I fully expect and predict that Gustafsson wins (arguably again). I scored it for him 3-2 last time and most fighters and fans had him winning too (BJPenn.com's poll was over 17K votes which had Gus leading 3 to 1 for him beating Jones, it's even higher now I'm sure). I saw a frustrated and beaten up Jones go back to his corner in the 1st round of that fight. He already started to look beaten, in shock sort of. He was already cut badly over the eye, outstruck, stuffed and taken down (for the first time ever in his entire MMA career). That's the absolute WORST way to start a fight and have to go back to your corner after. I think Alex's strength and speed really shocked him. He laughed pre-fight when both Glover and Phil Davis, even Alex himself, all said that he was the quicker of the two. He found out this was the truth (hand speed and foot speed) when they fought. When they did clinch he couldn't do anything to Alex. He shot very hard for some of his takedowns (watch round 4, about midway if I recall) and was stuffed immediately and with what seemed like no effort on Alex's part. It was like Jones was running into a brick wall when trying to take this guy down (Viking strength + great TDD apparently). He was getting outmuscled. Jones was getting his takedowns stuffed and nearly all of them pretty damn easily, nearly a dozen of them in total. Even when he had Alex hurt at the end of the 4th with that spinning elbow he followed it up with knees from the clinch that landed flush, Alex still didn't drop, and he attempted a takedown after this only to be shoved off and to the ground by 'Zombie Gus'.
Alex's strength and speed are pretty scary. He has great power too and I think in the rematch he'll sit down on his punches this time (certain punches) when he feels he's safe to avoid the TD (countering a strike for instance and using his timing to land a big counter punch). Don't be surprised to see him get the stoppage within 3 rounds. He's dangerous and a very fast starter (knocking guys down usually within the first minute). This alone can start to break some fighters right off the bat. He can take one hell of a beating and not even drop once, and he apparently eats knees, elbows, punches and kicks like "it's normal" (in my Anderson voice). Scary dude, crazy chin, and he'll be even more determined to not just beat but stop Jones in the inevitable rematch. He won't let it go to the judges this time. You can be sure of this.