Keeping to 1/4/12 pattern is good, but it would surely add a lot of fun and challenge to have something new like arena for 6/7/8 people.
I see several pro's in this idea:
- New strategy. 4 and 12 players strategy became banal. You bring 2 tanks and 2 heals for hardest content and you can switch tank\heal for dd in easier trials. For 6-8 it there will be more diversity in teams. Let the players find out the best team\roles.
- Better performance. The distance between 4 and 12 in performance is huge. In 4 mans content you can have 90 fps all the time while on some bosses it 12 mans fight you will get easily below 10.
- Something new. Old structure becoming boring and all chewed out for most.