A lot of existing raid guilds are worried because the new raid cap means that they will have to change their structure to meet the new requirements. Here is a listing of the options as I see them.
1. Stop recruiting and let attrition reduce the guild size.
All guilds suffer attrition. People leave for real life or other options. During the levelling from 60-70, you'll probably have some natural attrition, decreasing the numbers in the guild. This might leave you at the proper size, and it might not. If it doesn't, you'll have to use one of the other options
2. Cut the weakest people in the guild.
Identify your weakest players and cut them. This gets you down to the required size, and preserves the strongest part of your guild. However, it's painful to have to leave your friends. As well, this decision is ruthless and may cause a lot of drama.
3. Recruit more and run two raids.
If you recruit a few more people, you will have enough to run two raids. However, running two raids is a lot of administrative work. Splitting people up into an "A" Team and a "B" Team may also cause bad feelings, as people on the "B" Team will feel slighted.
This might be interesting in a winged dungeon, though. Team A could work on one wing, and Team B could work on another wing. The two raids could swap strategies as they defeat bosses, making it easier for the other team.
4. Introduce rotations.
Instead of each person raiding 5 nights a week, each person raids 4 nights, allowing everyone to raid, though at a lesser rate than previous. The advantage is that everyone remains together. Some people may dislike sitting out, though. Progression will also be much slower, as it will take longer to gear everyone up and for everyone to learn the encounters.
5. Set up an Guild Arena PvP team.
Take 4-10 people who like PvP and set them up as an Arena team. While the rest of the guild raids, the Arena team goes out and PvPs. Then when a PvP season ends, the Arena team takes their new toys and is folded back into the raiding team, and a new PvP team is set up.
Advantage is that the people who are not raiding are doing their own thing. They are upholding the guild name and reputation on the battlefield. As well, it provides a nice opportunity for people to experience PvP as a solid team. It gives people an opportunity to mix things up. Hardcore raid for a while, then hardcore PvP. There may be a hiccup in guild quality at the end of each season as people come back to the raiding group, as you have to gear and train the PvP'ers.
Personally, I think I would go with a mixture of attrition and PvP Arena teams. I think it would be the easiest, depending on the length of a PvP season. It gives people an opportunity to try different things and get different gear. Also, it'll give your raiders a chance to go off-spec, and indulge their inner Shadow Priest/Feral Druid/etc. to their heart's content.
As well, I am quite partial to the notion of having a team specifically carry the guild banner in PvP. A good guild has to maintain a certain reputation after all.
Thoughts? Any other ideas on what a raiding guild should do?