A few ideas which may help make Guilds more feasible with PlayFab:
1) Chat handled by Photon Chat
2) Allow Photon Chat Webhooks with Cloud Script for caching/saving. Allow Cloud Script functions to decide what to cache and what to discard.
3) Exclusively a paid feature, not available for free because of how server heavy it inherently is.
Personally I think the combination with Photon Chat could be a big help. Any type of string can be sent via Photon Chat, including JSON strings. This gives a lot of flexibility, it could be used to show a battle request, a trade request, or any other specific element. In combination with Webhooks and Cloud Script we could choose to always save/cache important messages over less important ones. Eg: trade request vs basic chat message.
I'm sure the PlayFab team has been looking at Guilds in-depth, but I thought I offer my thoughts in case they help.