an emulation of one of the following:
OB-8 (I know, there are issues here with Gibson)
EMS Synthi AKS (this would be insanely awesome)
Elka Synthex (such a good poly synth)
Roland SH-101 (would be good if they wanted to add a simple and cheaper synth to the line up... It would have to be "affordable" though. Like Way Out Ware's new "kickaxxe")
Roland System 100 (would be awesome)
Or if Arturia would want to try their hand at emulating something digital, a vintage sampler plugin would be AWESOME. Synclavier would be my first choice (with it's odd synthesis based on FM/additive as well as sampling), followed by an emulator or something of that sort.
Or perhaps rather than emulation, they could make a new synth with the whole "hybrid analog/digital" thing going for it. Make the oscs switchable between FM (synclavier style, or maybe a DX-100 style 4 op engine), Wavetable (ppg style... None of this interpolated smooth NI stuff, I want gritty sweeps!), samples (lo-fi, of course), or basic "retro style" additive (like on the oscar), and add an awesome analog modeled filter. All in all it would be a very good combo of early digital tech and analog filter (and pre/post filter saturation perhaps?)