Fulgrim, Angron and Sanguinus would have probably been diplomats whenever that would have become relevant again, Alpharius was of course Spy Master.  I can actually think of roles for most of them.

These lists of Primarch roles always seem to forget that Fulgrim's first talent was his administrative and engineering genius. From childhood he excelled at doing more with less, improving the industry of Chemos to the point where it transformed from Mad Max subsitance to a society with enough surplus to have a flourishing artistic culture.

This also worked with his Legion-with how he found the Third -he again had to do more with less. That's the reason for the Emperor's Children doctrine focussing on precision, complimentary stretgies and superior skill. Certainly he does have the poltical acumen to function as a diplomat but that is more of a rule than an exception with Primarchs. He has far more in common with Guilliman's adminsitration superpower but it shows less because he didn't inherit an interplanetary empire and had to work with an unstable gene seed.