We use Hero Game System, which is genre agnostic. We have used it to play low or high-fantasy games. We have also used it for steampunk, superhero, spy and space genre games. Hero Games does sell some campaign backgrounds that you can use. Also, quite a few people have provided conversion options from d20 games to Hero Games.
I would like to throw Fantasy Flight “Genesys” generic system in the ring. It’s very content light, but the potential to customize it how you want is extremely high. Tons of good fan generated content to pick through.
Well I have to clarify one of my recommendations now, I went and looked up Iron kingdoms to see if they had released new supplements in the last 3 years. They have abandoned their own rpg based on their wargames ruleset. Though many here might like it now I cant speak to the quality or gameplay of the new Iron kingdoms 5e content.
There are a couple I would like to play, and both are in my collection. One is High Caster (there’s an actual play on YouTube), and the other would be The Hero’s Journey 2nd Edition. Got both on Kickstarter, and both have, I feel, an “epicness” about them.