An important technique in genres such as ambient, deep techno, psytrance (amongst others) is the use of bandpass filters. With the harmonic richness you get from FM synthesis, bandpass filters can create awesome psychedelic and futuristic sounds, especially with LFO’s and envelopes. Unlike the base-width filter, which has no resonance, slope or envelope control, a bandpass filter on the main filter page would allow for a plethora of sound design possibilities. I don’t see any reason this should not or could not be implemented.
I believe adding a bandpass filter to the main filter section would be both easy, and useful. Given this feature is available in many other synths such as Operator, I see it as a huge disadvantage that the digitone doesn’t have this capability. If I had known this was not available on the synth I would have reconsidered my purchase.
For reference, the main synth in this track uses a sweeping bandpass: https://www.youtube.com/watch?v=sjl4orBmDL0