While I do love epic power metal/US power metal it seems like it wouldn't go long with MGS games--well the faster stuff would (I was gonna suggest thrash myself). I'd play it anyway, but the heavier power metal (Brocas Helm, Jag Panzer except for that one album after Ample Destruction, Rebellion from Germany, Riot, Sacred Steel) those all fit for like a Skyrim, Dragon Age, and general straight forward fantasy series that is less on whimsical high fantasy and more Conan/Elric/Imaro/Kane end of the fantasy spectrum.
The whole Viking Saga from Rebellion, Stormwarrior (which is basically Running Wild only focused on Vikings and a dude that sounds like Kai Hansen), and Bathory's Viking metal stuff was my soundtrack to Skyrim. Ran with Running Wild, Blazon Stone, Alestorm and Zed Yago for AC4.