Thanks everyone - it made me try to analyse what I like about those games. I definitely like the beauty of the Skyrim world, and the ability in both to travel about and get lost in side quests.
I really don't like a "camera hovering above" view, like in Assassins Creed, so I'll avoid any that only have that option. I need first person to be happy. I also like the ability to craft weapons and upgrade things - Skyrim armour and crafting, and Fallout weapons building is quite fun too. I enjoyed building bases in Fallout too, although not to the extent of some Youtube videos I saw - yikes.
Could be a choice between Red Dead and Horizon I think - I'll watch some videos.