Here's your key to factions Bungie.
Just allow faction vendors to offer daily, weekly, and random bounties along with an updated gear pool from Y1. Every few seasons maybe update the pool to offer ornaments on the armor like you did with faction rallies and like 4 new weapons.
Edit: If you were to bring back faction rallies just do like a Vex Offensive type thing where the faction with the most participants or kills wins.
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A mix is fine imo, as long as the factions stay the whole time and faction rally gets improved on.