Militaris Dominatus? Imperius Maxima? Something that makes them sound like they are the bestest, with a latin twist.
What is the base generic Primarch profile people are working from?
WS8 BS6 S5 T5 W4 I7 A5 Ld10 Sv2+ ?
When I did this a while ago I pretty much just took the Chaos Daemon Prince profile as the basic statline - the rationale being that the primarchs were analogous to his deamon princes. Then you apply the daemonprince template over the top of those that then became daemon princes (by referencing the stat difference between a chaos lord and a daemon prince).
EDIT: minus the monstrous creature part of course.
So the base primarch would be:
WS7 BS6* S6 T5 W4 I6 A4 Ld10 Sv2+*
*These being the only exceptions to the statline.
This is pretty much +1 per stat vs a normal marine commander. Then you would have some stat variation. What I envisaged was people like Dorn and Guilliman possessing the above stats as more 'generic' primarchs, where their strength lay would be in army augmenting special rules (Guilliman would have strategic assets coming out the wazoo for example), whilst people like Russ and Night Haunter had slight stat modifications to reflect their predilections.
Then you apply the daemon template back over that to get daemon princes, with someone like Lorgar having a basic profile like:
WS8 BS6 S7 T6 W5 I7 A5 Ld10 Sv2+
Monstrous creature, daemonic, 4+ invulnerable save (reduced from the sons of the emperor, lost their protection maybe)
With again the other more extreme variants receiving changes (like Mortarion being T7...)
Hellebore