Orcs Must Die! 2 has the often-condemned “DLC in the box” … of sorts. 10 levels from the original game have been re-done for the new game, which means you can play them co-op, use your OMD2 character with all its upgrades, etc. How do you get this? Own the original game. This makes it a bonus for returning players and an incentive for new players to try the first game (which can be under $5 with all DLC when on sale).
If you’re going to put DLC in the box, the polite option is to give it away to most of your target audience.