There could be some ways to make a horde mode; here's my idea of how it would work; it takes you Characters stats (this does NOT effect your actual characters world, it just clones him/her) you walk out of prison and for freedom you have to defeat X amount of waves/creatures, waves 1-5 animals, waves 6-10 creatures (giants, trolls) 11-15 humans, 16-20 easier but way more humans, 21-30 dragons.
or just a dragon hunt, you are in a forest( not a real forest in Skyrim, a custom made forest) and every 2 minutes a new and harder dragon appears, kill them quickly before you have an army of dragons (this will be like nazi zombies where you WILL die, but will have fun killing dragons in the process)