Please allow 2 things:
Custom tuning. You can already do higher or lower pitch by using a different sample, but what about relative pitch? OpenMPT currently only has equal temperament. Ratios relative to C are 2^(n/12). However, I may want them different; for example G to be 1.5 instead of 1.49830708 (2^(7/12)).

More notes per octave. OpenMPT only has 12 notes per octave. However using arbitrary number of notes would create issues with notation. Maybe add extra notes under custom names with user-typed ratio relative to C, and it would be nice to allow renaming existing ones and mapping keys to them. Or handle some values for "notes per octave" with set names - 17 and 19 can be notated using both flats and sharps.