One arranger feature I'd like to see is way to force 3/4 and 4/4 in the same timebase. Most of the time, you get a 1/4 note equivalency, so you go from a jazz waltz to a jazz swing, and rather than four beats in the space of three, it keeps plowing on at the same tempo...

A Bar=Bar button would be cool.
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An arranger is just a tool. What matters is what you build with it..!