You could tier it by diving up the perks and requirements instead of the current all or nothing system we have now.
Say a faction only has 4 members but is very active and provides a lot of fun, under the current system they'd remain as unofficial. With a tier system they could apply for a tier which would give them a section in the forums and the ability to upgrade a PoB core for a lower required ingame activity and/or number of members.
Basically you divide up the perks of the faction along with the requirements so as to encourage factions to aim higher.