ZWK
Footballguy
The biggest change is that ordinary touchbacks come out to the 30 in the NFL vs. the 35 in the XFL/UFL. That makes kicking teams more willing to just boot it out the back of the end zone. The average return in the XFL reached between the 28 and 29, which is within a yard or two to where the NFL touchback goes, and if it's the same in the NFL we may see a fair amount of intentional touchbacks by kicking teams that decide to play it safe rather than facing the uncertainty of a return. If the average NFL return makes it past the 30 (which is possible, because there are a few other rule differences, and talent differences) then we might just get a touchbackfest.
The other potentially huge change is that in the NFL the 10 tacklers on the kicking team can start moving as soon as the ball hits the ground, if the kickoff bounces. The XFL/UFL they had to wait until the returner touched it, or for 3 seconds after its initial bounce (whichever came first). That might just be a minor difference which doesn't change much, but it also potentially could transform kickoffs into something really weird if kickers regularly try to hit tricky line drives with spin which bounce in the "landing zone" and are difficult to field. (I've seen these called "long squibs" and "line drive knuckleballs"). Then a big part of the kickoff play would be about how nasty a line drive kick the kicker can hit and whether one of the 2 returners can get to it and field it cleanly. But it's unclear how well this approach to kickoffs will work.
The other potentially huge change is that in the NFL the 10 tacklers on the kicking team can start moving as soon as the ball hits the ground, if the kickoff bounces. The XFL/UFL they had to wait until the returner touched it, or for 3 seconds after its initial bounce (whichever came first). That might just be a minor difference which doesn't change much, but it also potentially could transform kickoffs into something really weird if kickers regularly try to hit tricky line drives with spin which bounce in the "landing zone" and are difficult to field. (I've seen these called "long squibs" and "line drive knuckleballs"). Then a big part of the kickoff play would be about how nasty a line drive kick the kicker can hit and whether one of the 2 returners can get to it and field it cleanly. But it's unclear how well this approach to kickoffs will work.