Currently all illuvials have the same stat potentials, so the only reason people would want a holo/dark holo is for their rarity, and they're not even that rare. I'm currently skipping over most holos I find simply because it doesn't actually feel that rare.
I think holos and dark holos should have a very slight edge over non-holos, for instance holos could have a 55% max cap on stats, and dark holos could be 60%. It could be a cool rare sight where a specific stat can go "off the charts" and past the normal graph (and instead of rolling 1/5 chance it's like 1/6). This makes an actual strength difference and more variability in min-maxing teams for later gameplay. It'd also create a stronger feel of some holos being ultra-rare vs. all of them feeling kind of ordinary.
This would create a higher ceiling in Leviathon as well but not game-breakingly so. You would have players with teams of holos with slightly higher stats that are worth much more on the market, but they could still lose to ordinary teams.
Ultimately this would also drive more fuel demand and usage because people would have higher stat illuvials to strive towards.