Maybe slavers should be able to enslave every populated location (even big cities like NCR when you have the firepower needed), enslaving it will result by the place occupied by slavers. It would be so hilarious to enslave Navarro and even the Oil Rig (this should include killing everyone inside). Or create a new location by using the Vault 13 (and maybe the Oil Rig) GECK, maybe kill off your tribe-mates, allying with the slavers instead.
But there should be similar option (a lot of populated places occupied by your faction) for the Rangers. I can think only of making a new one using the GECK from Vault 13. Rangers shouldn't kill off whole populated places (since they're good) and occupy them.
Well, maybe there could be a quest to earn the citizens' trust and then they could "let you create a new rangers base there", which opens a new area in the town (of course, filled with Rangers). In some towns, there could be even possibility to be fully controlled by (and still not populated only by) Rangers - maybe Vault City, after allying with everyone who is against slavery in the form of the "servant" system (or doesn't wants to fight to defend this). So in this battle some of the Vault City guards/citizens and some Rangers should be on your side and after winning the fight it should be controlled by Rangers. Maybe arrive in some city and (fairly) prove that the one who is in charge (some bad guy, of course, not someone like Marcus) makes some kind of crimes and take his place. But anyway, there will be much cities the Rangers can't fully conquer and the Slavers can - like NCR, Broken Hills, San Francisco...
I suggest that there is some kind of unique rewards when you conquer some place - like a weapon unique in some way, for instance a small gun that does the best possible damage for small guns to robots, an armor giving some of SPECIAL (other than ST, obviously), some unique perk, a way of relearn your SPECIAL (maybe some doctor which appears when you do one of the quests for conquer), some unique NPC(s), a way to level your NPCs (otherwise the unique NPCs will be limited - there will either be not enough time to level them, they'll be got too early or their leveling won't be like the other NPCs - Skynet has the same problem) and much more. I'll think of more (and some of these - like the perks) if someone actually starts doing this.
However, this mod will change Fallout 2 very much - changing the aim from saving your village to expand your faction, when you reach certain locations you'll be stronger than in a vanilla game when you reach them. But I don't think this should harm the game anyway.
Another idea: Be a part of slaver-ranger battles (after talking to some NPC), from easier battles for lower levels with slavers/rangers with worse gear to tougher battles, all of them accessed by NPC in form of battlefields. Also, there could be a perk (see the upper of this post to see how could this perk be gotten) which allows you to loot the members of the opposite faction's armors off their corpses. And there could be donations for your faction in form of weapons and armor (and maybe money), unlimited and granting you stuff (maybe xp/money).