How to create “diagonal wipe” effect in FFmpeg?
Basic syntax is

ffmpeg -i old.mp4 -i new.mp4 -filter_complex
       "[1]format=yuva444p,
           geq=lum='p(X,Y)':
           a='st(1,(1+W/H/TN)*H/D);if(lt(W-X,((ld(1)*T-Y)/(ld(1)*T))*ld(1)*T*TN),p(X,Y),0)':
           enable='lte(t,D)',setpts=PTS+D/TB[new];
        [0:v][new]overlay" wipe.mp4


This will unveil the new video over the old starting from the top-right.

What this basically does is add an alpha plane to the new video and then manipulate transparency so that when overlaid a wipe is effected.

Two of the variables will need to be replaced with numbers by you.

D is the duration or the wipe, in seconds.

TN is the tangent of the acute angle between the wipe frontier and the vertical. This is the ratio of the horizontal wipe speed to the vertical wipe speed. So, TN = 1 represents a wipe at 45 deg and TN = 2 represents a wipe at atan (2) = 63.4 deg.