If they stick to their pattern with how they handled Philosophy Tomes, they will retire Myth, allow you to convert them to Soldiery, remove the weekly Soldiery cap and introduce a new type of Tomestone which will be capped weekly. Also, the former Myth items will either become drops in dungeons, or purchased through other means (I believe several Myth items can be bought with Allied Seals even now, so this may be the permanent replacement).
But, there's no guarantee this is what they have planned at all. We probably won't hear about the plan for a couple of months yet.