In my opinion, what you have just described my good sir is a lot of the Bethesda games. Oblivion, Skyrim, Fallout 3, Fallout 4 (though I personally have not played it.) I've clocked well over 100 hours on those games and I still find myself enjoying them. There is always new content I am finding and if I find myself getting bored, just hop on steam workshop and find some cool mods or buy some dlc's. Lastly, this might not be entirely what you are looking for because it is more sandbox rather than open world, but I always recommend some good ol fashion mount and blade warband.