Well my suggestion is to add some chance to meet some random enemy (different enemy in different parts of map).
For example :
In some molerat spawning location on world map you could have a very small chance (say 3% or so) to meet molerat king with more hp and more stuff and giving you more xp.
In raider spawning location on world you could meet raider lietenant with better armor and a lot of raiders and he would have advanced or unique weapon.
In super mutant location on world you could meet super mutant (very hard) caravan with a wagon full of some stuff.
This could be implemented on all parts of map and could expand the pve.