I've suggested this a few times now;
They should have worked the solo rankings system so that it incorporates everything. A Ranking Roulette for PvP, essentially. That keeps everything relevant and allows them to add new modes for variety, rather than new modes to replace content... The existing PvP Roulette is nice, but it doesn't work since it is a daily bonus people queued for once then most resumed queuing for a specific mode. A Ranking Roulette would be constantly in use, since we'd use it to raise our solo ranking.
The Fold, Secure, Slaughter, and Seize would all need updating to accommodate such a thing though... We'd need final stats to be uniform (that's my way of saying add Assists to old PvP), and there are a few other tweaks they could make (adding Adrenaline to the really old stuff, general balance tweaks to bring them up to Lv60, etc.). Beyond that we'd just need the way rating functions to be more individual based, rather than the final result. Still have a win/loss factor in, but if you play exceptionally well, racking up plenty of kills but still ultimately lose? That's worth +rating IMO. Likewise, if you play abysmally and die a dozen times but your team still wins, that's worth -rating. Worried about how Frontlines matches are far longer than Feast? Don't be. Longer matches = more chances for assists/kills = greater rating reward. Shouldn't be too hard to balance...
I'd also suggest the rankings be separated by Job, since that gives us more opportunities for rewards (and better rewards; PvP Relic weapons) and essentially balances out the meta. Jumping on a particular bandwagon because it's effective just gives you more competition for that number one spot, where as (as of right now) something like Dark Knight likely wouldn't have much competition in ranking. Perhaps I'm overestimating how that would play out, but I'd hope it results in more balanced Job spread, rather than stacking Summoners/etc....
IMO, that's a better solo ranking system, and it's better for PvP in general... It keeps everything relevant and provides variety... Might be some issues with queues, since Wolves' Den requires less people, but with such a Roulette that "hardcore" PvPers will be using over and over, one would hope the "casual" can queue up for Secure or Seize and have the Roulette fill that queue up... As much as I like Feast (aside from the balls rating/matchmaking system and all the salt my Dark Knight is producing), I'd much rather have variety in PvP than this flavour of the month crap I've hated since Slaughter... Game needs a proper PvP Roulette, and rankings is the perfect opportunity to implement such a thing, IMO.