we essentially already have this type of battlegrounds setup in ToH mass battle, with the only difference being there aren't crystals to defend. with that in mind, why not set up this battlegrounds idea similarly to ToH, by using the allegiances to form the teams rather than specific guilds or what not. that way it's ensuring that no "bribery" would happen, as there are in each guild both light and dark affiliations.
also, making it limited to a certain time or certain days is good, and possibly require less ToH points to enter, if you feel the need for that as well (though i'm sure this would like to get away from the ToH aspect).
