A few ideas:
A GC-Unrestricted "fill queue" (random GC assignment, enables friends of differing GC's to ally together finally).
Change PVP Roulette to include Bonus PVP Exp./Wolf Marks.
Remove queue-based 72man Slaughter (all it does it cause the client to delay to begin a match in hopes of filling a 72man instead of multiple 24mans; 72man Slaughter could still occur in a 'guaranteed format,' outlined below).
Have, at minimum, one virtually guaranteed 24-72man Frontlines occur every hour of the day (possibly, alternating between Secure/Slaughter).
For even odds & equitability, these matches could be up to Light Party queue only & assign a random GC to each entrant/Light Party group.
It could work like Chocobo Racing, you'd opt-in & then it'd put you in regardless of player count once the timer expired. Latecomers could join-in up to a certain time into the match.
If 100 players opted-in, both a 72man & a 24man would start on time.
The remaining 4 players would be put on a hold (this' where the "virtual" comes in) until enough additional players opted-in, in which another 24man would begin.
Heck, it could be designed such that if that queue were taking too long, a prompt could appear on-screen notifying players that enough have queued to fill a Wolve's Den bout instead & if they'd rather like to participate in that in place of continuing to wait.
Client could even show how many active, guaranteed Frontlines are taking place & how many have queued for the pending one... so those 4 have a way of telling if it's just not going to happen for them right then (current FL queues should show this anyway, they should show how many are queued up for each individual GC even).
Lastly, SE should design a rectangular 8v8 map to help combat this rare scenario too.
Besides, it'd be fun, could do like Capture the Flag or something.
Also, revivify Wolve's Den...
Make Leaderboards & have Arena Seasons with special PVP armor/weapon rewards or something.
Have separate Leaderboards/Rankings for Solo Queue & Light Party Premades.
And do a Daily Roulette: Wolve's Den (solo) & Wolve's Den (light party) for bonus PVP Exp & Wolf Mark's.
NOTE TO SE REGARDING FFXIV'S PVP:
I know you guys get a lot of crap from people regarding its design... but I tell you, haters gon hate. I play a ton of competitive PVP games & have PVPed competitively in nigh every MMO in existence.
Yes, I did WoW PVP religiously for a while... and I mostly loathed it for reasons you guys legitimately solved. You've implemented things I've been asking for in other MMO PVP environments for ages.
PVP-Specific Actions & Traits as well as PVP-Specific Balance Changes chief among them. You guys are on the right track with this machine, you've just handled the queue's & restrictions wrong.
Your current system shoots PVP in the foot by making it unfriendly for play-with-friends (e.g. GC Restriction) & by severely hurting queue times by things like GC Restriction & multiple formats too soon.
I'm not saying remove GC Restriction anymore, but ease it up... let people who don't care about GC-centric PVP fill the ranks unrestricted or ally with their friends of differing GC's somehow.
There's merit to GC loyalty & fighting under your GC banner, so keep it sure... but allow those who could care less to have their positive social experience aligning with friends of differing GC's in Frontlines.
Lastly, it's utterly imperative that you untie PVP Rank from GC's, that you use some of the added character data from 3.0 to save PVP Loadouts by Job, & that you add a "Swap Alliance" Feature for PVP (so if one Alliance has 4 Healers but another has 0, 2 Healers can trade-out).
Each of these things highly dissuade people from switching Jobs in PVP which can be hugely detrimental (no healer alliances) & deincentivize people to switch GC's in a negative way (I hate pirates but my friends love them, just because I don't want to be Maelstrom or they do want to be Maelstrom, I shouldn't be locked from Frontlining with them... it's just silly, let us have the GC we want & still Frontlines alongside our friends of differing GC's).
I love you SE & please don't take this the wrong way, I mean this with all my love for the developers & for the game--but your stubbornness with regards to GC Restriction is killing PVP. I say this with utmost passion for the PVP you've implemented (I honestly love it, so much so that this must be said to you, because it's dying a slow death).
So please, for the love of PVP, reconsider how you're handling this.