As you seem to shoot weddings and groups, you don't need a long telephoto (what did you shoot with the 400, and do you miss it at all?)

Some ideas:
Ultrawide (10-18, 10-22
100mm macro (useful for ring shots, details, and when 70mm isn't quite enough)
or a wide-aperture prime like one of the Sigma f/1.4 arts (24, 35, 50), or maybe a 50mm f/1.2, or 85mm f/1.2.

Your existing lenses should give you a good idea which focal length prime you'd find most useful, if you did go that route.