I'm kinda curious why you see a problem with a cooldown on MR. It is not like Origin when MR was available only during events (a really stupid way to handle it!). Putting a cooldown stops those that make themselves GMs from spamming MR to go on killing sprees with no consequences. Yes, making venge recharges (why the frig to those exist anyway??!!) expensive helps. Why not just simply get rid of them altogether?
Thing is you can pop MR and get around all that for the whole guild. If you are killed by someone on MR, you don't see them in your hitlist so there is no venge available. But as said the system is broken when people can abuse it. My suggestion is only one out of all the possibilities available to them.