Stamina gain in chests. +X stamina gain for x amount of hours.

Perhaps we can see something like +1 - +2 stamina gain for 6 hours from the chest. We can play around with the numbers and how it is delivered (potion, unleechable buff, etc.).

Another idea: 1 free DQ reroll for that day

Would love to see multiple ideas being incorporated like what doom and shindrak have said above

While some of these sound good on paper, they won't do anything to revive GvG. GvG needs to be worth it to make other guilds participate. Increasing the ranges is one step towards that. We need better rewards to use the RP on + making improvements to the GvG system that others suggested (like # of attacks from your guild).

Notification System for the upcoming conflict

A notification system wouldn't help, considering you already get notified when a guild initiates a conflict with you (and when your guild does). Since GvG is centered around PvP, would you consider when players attack others to be notified? No right? The same applies here.

Reducing the offline time for AFK targets

This will essentially reduce the already low available targets. It might be okay if you increase the ranges more to compensate.

Expand Level Ranges

Excellent step in the right direction

Update RP Packs

This is what ultimately will drive engagement and participation.

Others have made additional suggestions, like medals for GvGs or opt-in with higher-tier rewards. Overall I don't think these changes will do much without the right incentives (RP, Medals, Etc.)