I'd love to see liches be more naturally and organically integrated into the game, and become more unique individually. Maybe add a rare chance for liches to spawn like death squads coming after you in normal missions, allow larvlings to spawn in more places and become liches more naturally in some cases, and let multiple liches be active at once, even if just in the background.
Of course, alot of this won't be very interesting until more than two sets of voice lines/personality and more than a hand full of ability sets are added. Right now, none of the liches feel very unique after the first few, and it adds to the high monotony of lich hunting, running completely counter to the idea of kuva liches being personal, unique villains for us.
For the idea of liches being long lasting foes, eventually getting powerful enough to command ships in railjack, I'd like to see the ability to kill liches and take their weapon as normal, but have them come back again at level 6+ with a more powerful weapon and better gear, and become as powerful as what was teased for the system initially. Perhaps at level 10 or higher, they can become a clan objective to take down, and everyone who takes part in the hunt at that point can then claim kuva/a weapon/a unique railjack avionic or weapon at the crimson branch, once the super-lich commander has been defeated.