I admit that I don't really feel strongly either way if Zero Command becomes playable in 2.3, 2.4, 3.0, etc. but, I'll play devil's advocate here and list some ideas for the faction that can maybe be built upon.
I must strongly mention this now, even trying to come up with ideas itself was difficult. Zero Command is the least detailed warlord kingdom of the major 5, (GM, EA, ZE, PA, and ZC.) not to mention that it was also the smallest. Harrsk got as far as Abregado-Rae from Kalisz before the Emperor returned, where he then got some territory in the Atrvis sector, and considering that Mustafar was the notable planet in the area, it wasn't an impressive chunk of space. So my suggestions are trying to stay within the realm of canon for as much as possible.
1. Deep 3 as a level three shipyard: Deep 3 was the name of the shipyard where Pellaeon found Dominion and Megador, which Harrsk had either acquired or built. Deep 3 was simply the shipyard itself, not a celestial body, so we can't have it as it's own space if that's the case, however, it's Darksaber of all things actually helps out in making Deep 3 possibly be a planet for the mod. The book says that Harrsk had a base in orbit above a rocky world near a red giant, where he was building Star Destroyers. Considering what little we can actually take away about Zero Command, I'm content with having this unnamed rocky planet taking the name Deep 3, especially since he was building capital ships there. D and M were likely still being finished, like the Night Hammer was for Delvardus, hence why Harrsk never used them.
2. Shockwave as a modified ISD: Shockwave wasn't Harrsk's flagship till 12 ABY, likely because he didn't begin building it till after Shadow Hand failed. According to Darksaber, the Shockwave was bigger than a regular ISD, and had more weapons as well. So much like how Isolder has a fancier Battle Dragon, Harrsk had a fancier ISD. Though, the fact that he had a special ISD built gave me an idea about Zero Command units...
3. Zero Command uses modified ships: Teradoc takes what he can get, while Delvardus has all the shiny toys, and Harrsk... well, based entirely off of the Shockwave, lets say Blitzer Harrsk is the one who puts a silencer on a flamethrower. Lets say, VSD's with Octuple turbolasers for faster moving damage sinks, Escort Carriers that replace their hangers for artillery and act like mini torpedo spheres, it fits Harrsk so well, he is a madman, let him have MAD starships.
4. Grand Admiral Martio Batch as a recruitable hero: This is the super obvious one, of course Batch was killed by his second officer, who took the Grand Admiral's fleet and defected to Harrsk, but the timeline of that event was never established. So much like Maldrood being able to get Syn in era 1, have Zero Command acquire the Grand Admiral after capturing a high value world in Era 1, something to impress Batch to defect himself, since he apparently just sat around in space after the TIE Phantoms were destroyed. Maybe Fondor? Since it was close to what territory Harrsk had.
5. Daala as a hero in era 4: My last idea. Daala did work for Harrsk, briefly, after her attempt to fight the NR was quickly foiled. For Reunification, have Daala join up with Harrsk in Zero Command if she is defeated or if the Imperial Remnant is wiped out.