Possibility to modulate the FM-4 matrix values directly.
What problem(s) would this feature resolve?
The modulation matrix on FM-4 cannot be directly modulated with other modulators and cannot have automation recorded directly. This makes it awkward to modulate these values or record changes in their values as automation.
How does this feature fit in Bitwig as a product?
FM-4 is a Bitwig instrument. Modulation is a core concept in Bitwig. Being able to quickly and transparently add modulators for device parameters and/or record automation of said parameters is central to how the software functions.
Is there already an alternative way to achieve this on Bitwig?
If you set the matrix values to the maximum they will need to be, then you can modulate and/or automate the oscillator Mod pots which determines the amount of modulation that oscillator should apply. The problem with this is that if you later on want to change the tone further by increasing or decreasing the value beyond the range currently determined by the matrix and the Mod pot, you’d throw out the other automation when you change the matrix value. The ability to modulate and automate these values directly would not only be more convenient but would negate these issues.
Could it be implemented using Bitwig components or APIs, without Bitwig team’s support?
No
Could it be provided by a VST or something else reasonably integrated with Bitwig?
No, unless you accept that you can’t do it in FM-4 and use a different synth instead.
Are there other products that offer this feature?
Many virtual instruments include a grid-like matrix of automatable and modulatable values. Europa by Reason Studios is a good example of this.