What should the next lette be? a z e b i y o

The letter C, because the pattern is this (I think):

Every two letters it states the vowels (a,e,i,o,u)

Then in between every two letters there starts a pattern of, saying the alphabet in order and saying the alphabet backwards.

In other words they merged these two lines:

A, B, C, D, E...

+

Z, Y, X, W, V...

=

A, Z, B, Y, C, X...etc.

then by adding this pattern of vowels you get:

A, Z, E, B, I, Y, O, (C), (U), (X)...