Tree type advice

Hi

I am having a native hedge of mainly hawthorn and privet.  I want to plant, every 4/5 metres a small/medium sized tree within the hedge so that I can create some depth to the garden.  I would use all the same variety.  Looking for a tree that has a good shape to it and maybe some Autumn interest.  Thought about Sorbus as this retains a nice shape and isn't too large.  I did something similar with a garden and used standard ligustrum, looked good after 3 years but not sure what it looks like after that.   Thought about field maples but a bit too large for what I had in mind.   Can anyone think of anything else that might suit.  

Thanks