well if you are talking entire server V server then there are a few things to think about.
Numbers- if one server has 50 people log in and the other has 10 people log in then it's pretty obvious who will win because on a straight out battle 10 can't beat 50. Nothing to say it has to be a straight out battle though, castle and defending structures can balance out that 10 to 50 but you wouldn't know the numbers before the match actually started.
Timezone- if it is like Siege atm where the match goes for a specific period of time then you need to consider the play time of the majority of people, siege is currently very regular and so people can organise their guild for a time that suits them but what you get is guilds just playing against an alt guild because there is no one else to play. If you had it going constantly then it probably wouldn't work because most of the time there would be very few people actually in the "zone"
Reward- if you make it anything like diamonds then people will just log in 20 alts to boost up their diamond pool, so that is probably not a good idea. If you make it something useless then people will be less likely to actually do it, some definitely would play it for the challenge but majority of people want a reward. If you make it character specific then it won't work either and having the same reward every time also wouldn't work.
Play style- since RoM has tanks, healers and DPS you might need to take that into consideration, if the style is a pure DPS fight then tanks will feel useless and be less likely to play. I have been in many G v G fights in other games and you can adapt play to suit, heavy chars like tanks run in hard while more squishy chars hang back and heal/dps from a distance, these are lost of fun but it comes back to numbers, 50 will always beat 10 every time regardless of char types. Now if you had structures that are guarded by NPC that then need to be defended that can work for the 10 but as I said before you wouldn't know who has 10 or 50 before the match starts.
So basically I didn't give any real ideas but just gave you things to think about =)