Usually when a traditional name becomes so common I try to experiment with other variations:
ChristophChistophe
CristobalKester
But I think
Christopher is actually lovely as a middle name seeing it now. Of these I love...
Dorian ChristopherJulius ChristopherLeopold ChristopherMathis Christopher
And I'd much prefer...
Maxwell Christopher nn
MaxThomas Christopher nn
Tom
I personally don't think the other four work at all, with the exception of maybe
Joris Christopher which I'm still unsure about.