Here's one I'd like to see.
An open world rpg based on Ancient Greece and it's myths. It wouldn't necessarily be actually set in a historical Greece(I think that would be a bit restrictive) but have heavy themes of it. Of course it'd have to have a good seafaring element to it as well.
There are tons of monsters and enemies that could be based off the myths. You could gain favor with particular gods by doing tasks for them, and if you please them they would grant you boons, perhaps with items, blessings, and abilities. You could also recruit followers of that particular deity if you get in good enough favor with him/her. However, you would also run the risk of pissing off one of the gods, and they would try to curse you, or send their followers and monsters after you. Imagine saving a town from some marauding monster only to find out it was X goddess' favorite pet and now shes bent on ruining your life. You especially wouldn't want to wantonly slaughter a god's followers at a temple or something. The gods would always be competing with each other on the mortal plain, using heroes such as yourself as proxies.
Additionally there would be several city states. I'd like a map around as big as one of the last 3 tes games, but instead of a bunch of "cities" that feel like walled off towns there would only be a few well developed cities, as well as a few smaller towns scattered around. You could do freelance work for any or all of them, or dedicate yourself to one city and help it achieve superiority over the others. Perhaps you could represent one of the cities in some sort of contest with the other cities. Or you could just wander around the country side either helping people or terrorizing them. In addition to monsters there would be bandits and marauders of course, and you could either fight or ally yourself with demigods.
Mainly I'd really just like to run around with a spear and shield, smiting Greek mythological creatures. There would be a lot of character advancement, with you going from zero to hero.