As I had, and we have already boiled it down to, large scale battles, not the simple "beat wave one, beat wave two, etc." What OP wanted was something along the lines of this but an actual mode, I would like it to be more integrated than a mode you pick from the main menu.
The Civil War quests aren't good enough from what I've read.
Here's what I want: Here is my personal idea for a horde mode. It takes place in a massive battle in TES history (yes, I'm a new blood). Players can choose whether they want to be attacking the city or defending it. Once they've chosen they could choose a various role to play. For instance: one play you could decide you want to be in sword fights with people and fight on the ground and fight in and around the city (defender or attacker), the next you could choose to be an archer and fire arrows as they charge the gates, kill people when they got onto your wall by placing ladders, and knock the ladders over (defender), or ride in with the attacking troops on horse back with a single handed sword (attacker).
I'd also be happy with a Roman gladiator type mode where you're thrown into a pit where you have to fight wolves, bears, trolls, etc. A 20 NPC (as in like captured guards, bandits, etc) free for all in an arena would feel much larger than it really is