Here's my list of missing chords in the IRP chord library. Those chords listed below can't be substituted with slash chords.
Some of them are really important. Especially the Minor 13th (including the flat 7th):

C2(Maj7) or CMaj9(omit 3): 1, 2, 5, 7 (can be used to play a Gsus(add3)
C(#4): 1, 5, 3, #4
C(b5): 1, 3, b5
Cm13: 1, b3, 5, b7, 9, 11, 13
Cm(add9): 1, b3, 5, 9 (for pop/rock/folk)
Cm7(add11): 1, b3, 5, b7, 11
Dim(Maj7): 1, b3, b5, 7
Cm7(add13): 1, b3, 5, b7, 13
C-9(add13): 1, b3, 5, b7, 9, 13
C7(omit 3): 1, 5, b7 (for blues/rock)
C (bass): 1
CMaj13(#11): 1, 3, 5, 7, 9, #11, 13
C(b9): 1, b9, 3, 5 (for flamenco)
C13b9(#11): 1, b7, b9, 3, #11, 13
CMaj7sus: 1, 4, 5, 7