As the Feast Season 3 pulled the attention of most PvPers away from Frontlines, 24 man matches became regular due to lower numbers queueing for it. Now that we finally have no GC restrictions, 72 man matches are frequent and common again, especially with the crippling changes applied to the Feast and the relic light grind going on.

Separating the mode into categories would effectively split the queues, and one or the other would be back to long waits, even if there wasn't this initial rush/relic grind/Garo event. This is by and large a good time to be doing Frontlines, even if it might seem a little more hectic. RNG has always been the wild card in these matches, but we have to take it with a grain of salt (or a whole shaker for that matter).

Lastly, if I recall, future PvP content is planned for more simplistic A/B, 2 team dynamics, and is high on the wanted list for both players and devs. I'm not shooting down your idea at all, but I don't quite think it'll pick up much speed.