Will the ability to do interpolating rather than just in multiples be added? 

For example, convert an 18fps cine film to standard 24fps without speedup.

At the moment I have to do 4x, then use Decimate in Virtualdub to remove every other frame to get the 72fps down to 24fps. Just would be easier if it do the interpolating in one step!

You need to interpolate to a bigger FPS you want and turn on the option "Create an output with ..." to do that.