New species are indeed difficult in this situation, but if the community asset pool is abundant [it is! I swear

], then there are plenty of options one can remix/recolor/retexture/kitbash to make something that looks fresh. Back in the days, in the tilemapping era, people were going crazy at recoloring, retiling, returreting and reworking existing assets to get something new and provide their custom factions with fresh identities. Sadly, most of those projects never got finished and released... I miss those times.
This opens at least an option to have GTVA fracturing scenatio or an original Sol-based faction.
Does this require effort? Yes. Does this require modpack-making skills? Hell, yes. But it's worth it. I built several medium to enormous modpacks in my life based on conviction, that even if reusing the same canon designs over and over again may provide entertainment if the writing and worldbuilding is good, it does not bring anything new for the eyes... And sight is dominant sense of bulk majority of humans.
But the pirate/mercs have another problem: They are pirates/merc, like Trashman said. They have no right to roll enormous warships and hordes of fighters into the battlefield, vastly limiting possible options. Experience from Shadow Genesis made this clear for me. An enemy should be at least NTF-sized, with access to economy, logistics, command structures and stuff. Both HoL and NTF feel small, but even HoL alone has access to multiple destroyers and more resources, then modern US. NTF on the other hand is actually a fully-fledged, interstellar superpower, controlling multiple fleets, several star systems and has capabilities to wage a war against a stronger neighbour. Therefore both generate political environment to FRED cool missions around.
There is, of course, nothing wrong with setting the scale small and focus on civilian side of things... But most of the campaigns like that, like Twisted Infinities, Friends and Foes and also New Path, decide to go for big things anyway, generating the issue described in the op. This style of campaigns creates very limited creative space for interesting gameplay. I personally see no need for limiting myself so much and prefer to put my creative juices into larger conflicts because plots of larger conflicts provide me with all the creative freedom I need, without risks of entering "unbelievable" territory.