Fic search
There's a pair of fics that swap Keitaro and Negi, with one focusing on Keitaro teaching at Mahora, and the other focusing on Negi being manager of the Hinata Sou, though I think Negi is still a mage. Not sure about Keitaro. The fics were by the same author, and were meant to take place in parallel with each other, and crossed over at some point, with Kei's students meeting Negi, and pactios being mentioned (I think there was something about Kei not wanting to pactio with Negi, which is understandable; not sure if they had to pactio anyway, but I think there was something about exchanging blood being an option).
I can't recall whether the fics were all that good, but it annoys me that I can't recall the author or titles.