People have wanted vehicles other than bikes for a while, but it’s been said that it’d kill exploration, so here’s my suggestion.
In each area, put an “Artillery base,” similar to the one at the end of FINX rising. If you have a vehicle that’s bigger than a normal bike, you’re going to start to get bombarded as soon as you enter that specific zone - maybe a few different bases per zone, each covering part of it, depending on the size. That way, you have to explore the area on foot/bike at first, but once you do manage to clear out those artillery bases, you can bring in a bigger vehicle for faster exploration, such as hunting down rivals or completing smaller side quests you missed, or just going to a town you never visited because it’s super far off.
These would add new places to explore/destroy using existing assets, help give a better sense of progression (you’re clearing out areas of the island and making it safer) and making any future vehicles less OP overall.