Allow cross-realm raiding of current tiers, but only a max of 20% of the raid (2 people on 10man, 5 people on 25man) to be from a different server than the raid leader's server.
1. Keeps guild and server communities in-tact.
2. Lets people play with their friends in current tier.
Win win for everyone!
As an added benefit, I think it would do far more to solve the 25m decline than Blizzard's thunderforged system. It directly makes the logistics of 25mans easier rather than just incentivizing those guilds to deal with harder logistics. You can do a "25man raid" with only 20 people. 3rd party websites (like openraid) will supply plenty of people you can always use to fill in.