EDIT: Also, on a side note, I'm going through all of Angel Beats because I figured I'd do a mission based in that universe. It helps that my local library had it as a loanable item... and that I'm borrowing it. Now, with all that said, I'm thinking it'll be after the second mission I write... while keeping in mind that things are bound to get super sticky at speeds faster than teleportation.
This also ties into the bit on using the TSAB for this... I had a thought that one of the TSAB's best soldiers (unsure if it's Nanoha or someone else) went MIA, so they decide to send a lot of feelers for info on where said person is, eventually leading them to the Grail Works LTD. Of course, due to circumstances, the TSAB ends up contracting the Grail Works while Kurai is out of the office... so someone else has to locate the missing soldier. This leads them to the world of Angel Beats.
...no idea how the story will go from there on, of course, but still!