However, all factions that exceed 8 members (not characters, accounts) must choose an ID. Minimum number is 3. ID doesn't grant control of a system, but rather control of a station. And other stations and planets of same faction, but this control is rather vague and doesn't grant any specific rights.
Next, faction with 5 members or more might buy home system for 300 million credits.
System battles
This should just be simplier. How - I don't know, what's obvious is that attackers should respawn away from defending base and there should be a referee who will count number of respawns. If this will work, battles gonna be ok.
For now, at least, cause there's no model of system control in action.
New bases
I have to say no. There won't be new custom bases... except for...
The future
This is Discovery 4.81 thing. A simple, but nice idea that worked for other mods. Clan systems. We call them factions, so they are going to be faction systems.
Basically, it's a simple system with few objects, making it expandable. This system would have one or two bases and a guardian faction - NPC faction without any specific name. Another solution: it would have one of the existing NPC factions. For example, make a system near Frankfurt that would be a Bundschuh system. Guarded by Bundschuh weapon platrofms, stations and NPC ships. Not simple stations and NPCs, but those who can really hurt players and become a part of system defense.
For this model, base owning is possible (but limited to 3-4 per system). As faction systems are 'apart' from original FL and jump holes to those places are away from patrols, lanes and other objects, we might consider them a private place. This will allow to release patches that would change relations between different factions, build new bases etc.
This solves two problems. First of all, original systems are left intact. Second, faction control won't spoil normal gameplay for everyone. Faction systems won't contain any exclusive items or ships. All items available there will be accessible in other, neutral places like Omicron Delta, Minor, Theta, Gamma etc. So there'd be no more arguements like 'ah, you are Phantoms so don't come to our place' - everyone who's not your ally and entering faction system is an intruder and might be shot at. Should be shot, actually.