Asides from 'themed' level packs that come with new tools or toys, IMO, more levels from MM are redundant. The community is providing amply.
There've been a number of updates, the MGS pack, and a bunch of stickers and costumes since launch. If anything I actually thought the speed with which the MGS pack came surprised most people (it coming by Christmas IIRC). I do agree the time feels right for a new 'big' pack, but I've a feeling it is coming very soon..
Of course I'd like everything to happen faster..but we know what they're working on, we know it's worthwhile (i.e. the online collaboration tools, the website etc.). It's just a matter of having patience and not being spoilt