Other queries awaiting answers aside, I wonder;

How will they determine which side you're on?

I mean, obviously it's Maelstrom v Twin Adder v Immortal Flames, but doesn't that royally suck if your friends are in a different Grand Company? If your friends haven't PvP'd yet, it's fine, but if you have any progress in PvP rank you're certainly not going to want to reset it...

I'd hope they primarily base it off your Free Companies Standing with the Grand Companies, but that seems unlikely given how the PvP Profile is set out... Tying it to Free Companies would also make sense (assuming its a persistent zone rather than the same as Wolves Den) with regards to the future Primal Summoning content; your Free Company takes down a Primal, does something (they've never explained what), and gets to Summon said Primal. That would be an absolutely awesome thing to do in Frontlines, in fact it's the only place I can really see it working. Since it's server based the only other content you could unleash Ifrit on would be Treasure Maps? Guess that might change in the future, but it would still be most fun in Frontlines, IMO. Obviously that wouldn't work if Free Company members are splintered across different Grand Companies in Frontlines, though...

Perhaps they could remove the rank reset on changing Grand Companies, but in my opinion that would be a mistake. Well, only if Frontlines turns out like I imagined, since if it does there would need to be something to stop you just switching to the winning team...