The stock MvM maps & missions are fun but I guess a problem with MvM as a game mode is that your opponents (the bots) only do what they're told to do. In PvP much of the fun and longevity comes from playing with other hoomans who individually improve over time, and develop strategies & team work.
As a quick intro how MvM works is that besides the map which provides the game environment each mission is defined in a population file which have a
.pop extension. The population file defines the waves, i.e. what bots get spawned & when. To get more variety from a MvM mission you could change the map, or the population, or obviously both.
Even though the game mode is barely a month old there are already quite a number of community made maps, and many missions! Too many to actually review in one post so I'll mention just a few highlights and link to useful repositories.
Probably my favourite missions so far are LittleBigWorks, a mission for MannWorks, MannWorks Garden War and Sleepy Coaltown. They're all reasonably easy but some of the bots are absolutely hilarious. I won't say anything more so as not to ruin the surprise but strongly recommend you try these missions.
MvM DeGroot Keep is one of the funniest maps around. As with traditional DeGroot Keep it runs in Medieval mode so most classes can only use melee weapons with the exception of Demoman, Medic and Sniper. In it's default config it's quite easy but still fun.
MvM Manndarin is probably the most polished of all the custom maps I've tried – it's about on par with the stock MvM maps advanced difficulties, you'll need a decent group of 6 to finish it.
MvM Gateway is a very challenging map. I've only finished it once and failed many, many times on it's final two waves.
One thing to note is that the community made maps are quite buggy and will crash your server regularly. But on the whole they are good enough to be playable, and fun!