I am going to go with a street level campaign and I want to start with a series of scenarios where the players are only on the periphery of runs. The players are D&Ders and I would be surprised if more than one of them had read any spy novels. The goal is to introduce spy tradecraft such as dead drops, paying someone to provide misdirection, safe houses, caches, clean weapons, muscle for backup, preliminary steps for an extraction....stuff you pay the fixer to do but go behind the scenes and have the players start out by taking care of little jobs and then build into real runs.
I am throwing this out there to try to build up my inventory of tradecraft ideas. Feel free to brainstorm any service you would pay a fixer to provide.