I think it would be really cool to have theme battles like Noxus vs Demacia or Ionia and stuff like that. Summoners would vote on one faction and then they would have to play only with champions representing this faction! I know some factions have limited champions but I think riot can find a way around that!
vs 
, 
vs 
Team Ionia would be Op...
Top: 
, 
, 
, 
, or 
Jungle: , ,
, or
Mid: 
,
,
,
,
,
, or 
ADC: 
or
Support: ,
, or 
Lol. Lotsa power from Ionia.
I voted maybe. If it's just an order SR game with Demacia champs on one side and Noxus on the other, then it's just a gimmick. It might be fun for a lark but it wouldn't have any longevity. (I've played on an all-Yordle team and it was delightful but only because it was novel.) Maybe if they were keeping track of how many / what percentage of games each faction won and released a summoner icon for the winning faction, it would drive enthusiasm for a little while, at least.
Fanatstic idea!!!!
or maybe skin themed battles ? like the pentakill team (
) vs team pool party (
)
I believe that this would be a great idea as I remember playing the first DoTA and they had a light versus dark game type going on. I remember only that sometimes you wouldn't be able to play who you wanted, but also I think that being able to join a team as a specific faction instead of being random would make that easy to overcome.
i think a good void team could be like 
top 
jung 
mid 
adc and since theres no great sup pfor the void i guess 
would work
The Evolution vs The Resistance.
