I’ve wanted this a long time too, and have even employed hacks to my instruments.xml file to create instruments and modifications not available within the program but …
Daniel has said previously this is much more complicated than it initially appears. Dorico will need to know what the open strings are for a custom string instrument for example. When non-percussion staves with systems other than 5 lines are allowed natively, there will need to be a way to specify the tonality system or create custom clef configurations to accompany them. Obviously custom transpositions, range, and instrument family will be needed too. Dorico will need a way to specify if the instrument is a rhythm section instrument for chord symbols or a vocal instrument for dynamic placement. Other elements like # of staves, barline span for instruments less than 5 lines, and probably other things would be necessary too.
I’ve griped about this as much as anyone probably LOL, but I understand there is a heck of a lot more involved with wanting custom instruments than changing a name and a VST sample, both are which easily accomplished now. Daniel has said this won’t be forthcoming in 4.0, but hopefully in 4.5 or 5.0 we’ll finally get a completely implemented instrument designer that incorporates all of the elements mentioned above.