Alternate world battles can be brutal once you write and do enough research, even inspiration flows.
While if we're talking, irl, just being mortal and the common issues from reports of battles, etc., are horrific once you are in there.
Copyright is just a minefield waiting for someone brave enough to enter it, while the public domain is best to avoid the BS from companies, while any series under creative commons, ask the author first.
A rare few of us are sick and tired of companies, so cc route so people create what they like...