We won’t have any drum pattern library features in this first release, but you should be able to easily maintain a separate project containing your favourite drum patterns (e.g. as separate flows) so that you can copy and paste them into your current project.
In Dorico, because each percussion instrument (e.g. bass drum, snare drum, every tom-tom, every cymbal) is actually a separate instrument, it’s easy to copy the music from just one component of the kit to another instrument. In Layout Options, you would change the percussion presentation option from five-line staff to show individual instruments, which will make every instrument in the kit appear on its own individual single-line “staff”, at which point you can then copy and paste from that staff to another bass drum held by a different player.
As for MIDI and MusicXML import, we are doing the best that we can. With MIDI we assume that the instrument is using the General MIDI drum set, and then we import accordingly. With MusicXML, different applications export different amounts of percussion data, so the results can be variable. Again, the neat thing is that because each instrument is imported as an individual instrument and then combined together into a kit instrument, you can easily change a single instrument within a kit instrument, or change the staff position where that instrument appears in a five-line staff, etc., completely dynamically.