I love games with horde modes. Call of duty black ops, Mass Effect 3. It’s addicting to unlock different weapons and classes to find that perfect combo that you enjoy the most. But after while they can grow stale with a lack of content. So why not a full game with different time eras ? A hand full of maps can be dedicated to sci-fi themes with alien races and future weapons and another set of maps can dedicated to olden times with swords, shields and magic spells. Adding something like this could bring alot to the table and sounds like an awesome idea that I for one would be all for. Thoughts?
that sounds awesome 
Something like Left for Dead?
No mention of Gears 
Gears 2 practically reinvented Horde modes and started that craze.
I think the primary reason people have avoided what you’re suggesting is making a new weapon in CoD or a new class is a “minor” change, but revamping all the graphics and all the weapon mechanics for each “era” of a game is a substantial amount of work. Not saying it isn’t a good idea, just saying it’s a huge project compared to the “value add” CoD gets on taking their base game and designing a substantial mod around it. Certainly a large scope project, but it sounds like it would be fun.
Chivalry has a horde mode - the only one I know of that uses melee horde mode (probably others that I am forgetting).
Halo’s Firefight was kind of fun too.