I'd like to see an active crossover as well. The features I'd like to see lean more toward monitor/subwoofer use. In fact, I guess I should really say that what I'd like to see is a active crossover/subwoofer control box:

6 or 12 dB/octave high pass
18 or 24 dB/octave low pass
High & Low level inputs
High & Low level outputs
Stereo and summed mono (low level) outputs
Adjustable Gain
Adjustable phase 0-180
Adjustable rumble filter

But a straight 24dB/oct LR 2 way stereo/3 way mono active crossover with fairly wide frequency range would be useful as well.