This is an off-the wall idea...but the something like this could work. Instead of Luna just being the trusted advisor and exposition-kitty, she could be working both sides.
This would explain why Queen Beryl was so eager to execute her own minions in the anime. They were only a means to an end to flesh out the reincarnated souls she was interested in. It would also address the problem that as the series wore on how meaningless Luna became after the initial arc.
But when would LunaBeryl reveal herself? When Usagi was revealed to be the Princess? Or when Artemis came into the scene and him admitting he has no idea who Luna was, as she never would have been part of the Silver Millennium in her feline form. Either way, without Luna being there to support Usagi it would have made those weeks without Mamoru all the much harder on her, having to deal with both the abduction of her love and the betrayal of her closest feline friend.
It would be an interesting spin on the Witch-Queen and her cat familiar, at least.