I've heard about the map only allowing a certain amount to be revealed, but I've never seen it happen. But I only play 8k and probably never had more than 40-50% revealed in any one game, so that may be why. But something like that might be nice. The only thing about it is that I don't know that many people would actively try to do it, so it would just be free experience for doing what you do anyhow. As you say, it would be small amounts of experience. You'd get more by killing zombies and not have to run all over the place to do it. Maybe it would work better to do something like a larger amount of experience just to visit each trader, making you want to go to every town. Even people who do the Open Trade Route quests may often only visit those 5 traders and not bother going to other towns. But that only gets people to go to other towns. It doesn't get them to explore the whole map, which is what I think your hope would be for this suggestion.
There could be a new achievement for visiting a certain number of POI in a single game - 300 or something large like that. Visiting meaning you are on the POI. I don't think there would be a good way to count how many POI are cleared because you can't see if you miss killing anything if it's not a quest. But visiting could work. And with a large enough number, it would get people to really explore a map to get that achievement. Or it could even be a challenge that people could do each game. Perhaps part of a new challenge group that is about exploration.