Reaper uses a very odd pan taper. To bring it more in line with other DAWs I'd suggest adding a pan taper selection to the current pan law selection (as a second listbox. maybe the compensated/uncompensated could also be turned into a checkbox, which would halve the length of the pan law list -- and be slightly less confusing for beginners methinks).
The pan tapers would be:
Reaper (which should be the default, as this might otherwise break older projects / workflow for new projects)
If you are about to complain, that both square-root and sinusoidal tapers are fixed to a -3 dB law, they can easily be extended as so:
P = Pan amount [0:1] (0 = left, 0.5 = center, 1 = right)
L = Pan law (reduction at center in dB, must be unequal 0dB. example -6dB => L = 6)
For square-root (or maybe it should be called exponential):
Ch1 = (1-P)^(L/6)
Ch2 = P^(L/6)
For -3 dB we get the standard square-root taper and for -6 dB a proper linear taper.
The sinusoidal law could be adapted in the same fashion:
Ch1 = sin(0.5*pi*(1-P))^(L/3)
Ch2 = sin(0.5*pi*P)^(L/3)
For -3 dB the result is a regular sin/cos taper, and for -6dB we get a squared sinusoidal taper.
Other tapers would be possible as well, such as piecewise linear which would approximate how actual potentiometers are built.
See also: http://forum.cockos.com/showthread.php?t=49809