Take the X-Fi Titanium HD I have right now and...
-add A3D 2.0/3.0 support (Creative buys out Aureal, only to drop support for their DirectSound3D extensions in favor of their own EAX? Ugh! I don't care if they were abandoned earlier than EAX, it still means I have to build a Win98SE-era computer with an Aureal Vortex2-based card to hear those games the way they were intended, wavetracing and all.)
-add HDMI 1.4 audio output and input (The Auzentech X-Fi HomeTheater HD is based on the slightly older HDMI 1.3, so that's not quite ideal.)
-add XLR microphone inputs with a 48V phantom power supply (in other words, add a professional-grade pre-amp)
--EDIT: FOUR XLR inputs for soundfield microphone compatibility
-allow simultaneous headphone and RCA jack output, as requested above
-allow adjustments to the CMSS-3D Headphone HRTF
-maybe add a Stax/electrostatic headphone amp (conventional dynamic/ortho headphone amps are almost useless to me since they cannot drive my SR-Lambda)
-offer an external USB or ExpressCard variant that retains the EMU20k2 DSP and its gaming features in addition to the usual internal PCI-Express cards
-restore Dolby Digital/DTS decoding on the S/PDIF input, for console audio recording purposes
-eliminate the mode switching and make all features available under one sound card mode
-trim down the driver bloat and make it all more stable (though the Titanium HD drivers are more solid and stable in my experience)
That could very well be the ultimate sound card; too bad it doesn't exist. Still, I like my Titanium HD as is.