If you want to "a complete rewrite" you could use Delphi, sure, but why ?
The Plugin is developed quickly without limitations, all the limitations are with the BBW4 side and waiting for a communication line to be added.
The deal is the "OLD" code that has to be modified, I would say that is where the bottle neck is, that these things aren't just "DONE" as they would be if it was ALL done in the Plugin code.
The example is, if it can be done in Reaper without limitations, any time signature and more chords per bar, with some Lua script the same can be done moving the basic code to the Plugin and improving that rather than having to go through all the code in the old Delphi bbw, with a lot of code from the DOS days that is limiting things. Start afresh with the basics, get away from 1990 8.3 ACHYBRKY.MGU _SLOBLZ1.STY in the Plugin.
"For TimeSigs higher than
4/4, each bar will be spread out
over 2 or more bars (eg. 5/4 will be
written as 3/4+2/4)."
The RealDrums can be easily made to be Generated directly in the Plugin the same way it was in Reaper with a script.
There could not be that much code to generate up the RealTracks from the RealTrack files, they have all the info you need just like the F5 UserTrack settings, the Plugin could also easily do the UserTracks without the need for BBW4.
Like I can modify some Delphi forms and compile them back into the exe but I should have to do that.

RealBand is also a front end of the old bbw2 code it communicates with in the background and is limited to workarounds just like the Plugin.
It will be good to see more tracks in Biab along with re-gen any bars, but the other major things like mentioned would maybe be better/easier implementing in the Plugin, also implement play from RAM in the Plugin ?
I see the future in the Plugin that is not limited to workarounds and therefore can develop unhindered.