Adding new factions would require a total rethink of FW, which at its heart is a capsuleer proxy war mediated by CONCORD. Participants are recognized CONCORD member states, who submit to rules limiting warfare to pre-determined areas in order to avoid the destructiveness of all-out war. The idea that the pirate factions, for instance, would submit to CONCORD's rules and fight only on a pre-ordained strip of systems is laughable. There is also issues of sovereignty, in that CONCORD doesn't recognize the Intaki as a state separate from the Federation, or Mordu's Legion as anything but a well-organized mercenary group.
So, here's that rethink.
Step One: End the Empyrean war. The capsuleer militias have been at it for years, with no conclusive result.
Wild swings in the Caldari's favor after an initial year were completely reversed in the second, and the front lines have settled into a Gallente occupation of much of Black Rise. However, the Caldari Titan still looms in Luminaire, and the six month occupation of Placid has left a lasting scar in Federation politics, highlighting divisions between the Gallente core and the disenfranchised Intaki. The Intaki council has already turned to Ishukone and Mordu's Legion for security, forging an independent route regardless of the outcome of CONCORD's proxy war.
Tibus Heth's rule looks less stable with the economic crash caused by the loss of the Placid systems and all the investments he urged his supporters to make in occupied Federation space. Further loss of many of the Black Rise colony systems, once touted as the future of the Caldari State, will give his challengers a chance to undermine him and push the State back into a less centralized balance between the major corporations. The one clear gain Tibus Heth can point to is the reclamation of the homeworld, a tentative victory assured only by the guns of an orbiting Titan.
On the Minmatar and Amarr front, fighting has been concentrated in the same handful of systems, resulting in much destruction and loss of life for relatively little gain. A recent Minmatar push into Huola has been reversed and the Amarr hold a tenuous advantage with several Minmatar systems under control. Some systems have exchanged hands several times in recent weeks, often leaving planetary invasions stranded without orbital support. Troops and generals from both the Republic and Empire have grown tired of their fate dictated by the capricious nature of podder warfare.
The Empress (in between orgies) seizes on the opportunity to solidify her power. She can lay claim to having vanquished an Elder fleet with God's intervention, then rallying her people and holding the borders against an unprecedented assault on Amarrian space. The stalemate is trumped up as a victory, an Empire holding strong against heathen hordes backed by Gallente financiers. To an extent, the Empress hand is forced - spreading rumors of her cloning, debauchery, and errant personal behavior is forcing her attention towards domestic threats. Rival Houses shocked into compliance by her sudden rise and the advent of war are regaining their feet and reassessing the situation. Imperial politics return to cloak and dagger, sectarian conflicts, and the rivalries between centuries-old families. The Khanid take the opportunity of the Empress' vulnerability to strengthen their autonomy, forging economic ties with their Caldari allies.
The Republic has its own hands full, with the influx of freed slaves from its brief advances into Amarrian space, the political ramifications of the Starkmanir rediscovery, the revelation that the Elders exist, and the prospective incorporation of the Thukker. Still a new and fragile state, the Republic has been particularly drained by warfare and agrees to a tentative cease-fire without submitting to a permanent peace treaty. The border between Republic and Empire remains heavily fortified, with live fire incidents being not at all uncommon. The multiple crisis' of the Republic's short existence has renewed many of the tribes' reservations about the Gallente-influenced government, and the Minmatar turn inwards. Its score with the Amarr left unsettled, the Republic refocuses on shaping its own identity.
Step Two: Everyone gets a militia!
Despite the bitter fruits of the Empyrean War, the military benefits of a capsuleer militia have become indisputable. Repeatedly, capsuleers have swatted away conventional forces lead by baseliners, fulfilling military objectives effortlessly whenever there has been an absence of enemy capsuleers. Many organizations, large and small, start to directly employ capsuleers for their security and defense. Quite a number have been doing this already with limited contract work and agent missions. These factions start opening their ranks further, employing capsuleers on a semi-permanent basis to patrol their borders, maintain their claims, and secure their operations.
The Guristas start directly employing capsuleers to expand their raids into State space as well as defend their established territory from rival pirate groups moving into their business.
Angel Cartel continues to its operations in Curse, employing capsuleers with the lure of its unique technology. Having already shown a history of conquest with the destruction of Feythabolis, the Cartel is unrepentant about expanding its power base.
The Blood Raiders, driven by their hatred of Jamyl Sarum, start drawing in capsuleers as tools for their agenda. The already eccentric nature of many podders, known for their corpse collections and willfull disdain of the lives they take, offers the Sani Sabik a receptive audience for their blood rites.
The Serpentis Corporation faces continued attacks by Federation law enforcement authorities, seeking to crack down on destabilizing and illegal drugs. In addition, they find a rivalry with the Syndicate, the Intaki expatriate organization that dabbles in many of the same illegal markets as Serpentis does. They turn to capsuleers to protect their territory.
<Add blurbs for Sansha's Nation, Khanid Kingdom, Ammatar Mandate, Mordu's Legion, the Syndicate, and all the other 'small' factions of this bloody huge universe>
Step Three: All low-sec and NPC nullsec becomes a potential battlefield for faction battles, with dynamic politics between the factions.
<Insert sweeping generalizations of the complex mechanics and nuanced rules needed to make this work>
???
Profit!