Honestly, I don't know what makes such games fun. Exploring is part of it, but there's not much of that in Destiny, if Old Russia is much to go by. My favourite open-world game ever is Skyrim, mainly because of the sheer volume of content: there are quests and places and stories in every little part of the map, the total opposite of Destiny. I think a good start for the latter would be much bigger maps. I find it difficult to believe Old Russia is the same size as most of Reach's Campaign.