A polymorphic language

Here's an idea that I'd like to play around with in the near future. I'm
defining a polymorphic language as one where most morphemes have several
possible allomorphs, and it's not always predictable which one will occur
in a given situation.

To do this, I'm going to start off with an ancestor language with a more
simple and predictable morphology. The phonemic inventory will be

p t  k ?
f s  x h
w r j
m n  N

i  u

  a

and the allowed syllables will be [C]V[L][n]

However, each phoneme will have more than one allophone, eg

/hi/ = [Ci]
/ha/ = [ha]
/hu/ = [Wu]

Then, I'll introduce sound changes that turn the different allophones into
separate phonemes. That should leave me with the effect I desire.

I'd welcome suggestions for what allophones different phonemes should have,
and how to break allophony.

Pete