A feature that allows for setting tempo in multiple places would be interesting to give more freedom in sequencing.

For example if one would want to make a piece inspired by Piano Phase from Steve Reich one would have to make two instruments playing the same phrase but one is slightly faster then the other. This could look something like:

list phrase random(16)

set tempo 120 
new synth sine note(phrase 1) time(1/16) // uses tempo 120 bpm

set tempo 121
new synth sine note(phrase 1) time(1/16) // uses tempo 121 bpm

This however requires a rework of mercury where instruments use separate clock sources instead of a global transport. Another option could be to allow for some tempo/time multiplication factor within the time() method, for example as a third argument.

list phrase random(16)

set tempo 120
new synth sine note(phrase 1) time(1/16) // uses tempo 120 bpm

new synth sine note(phrase 1) time(1/16 0 1.00833333) //uses tempo 120 but multiplied by 1.008 to reach 121, the 0 is the offset argument that we don't use