What should I plant here?

Hi all, got this area which I'd like to improve by removing the Acer (this will be placed in another part of the garden) and planting something that is more likely to fill the area. I've started removing the plum slate. There's a thick layer of it. My intention is to improve the soil, get rid of the slate and replace with bark chippings.

It measures 10ft at its widest. However the box hedge has been infected by box moth and may need to be removed at some point so this would help increase the space a bit. Just outside of that area to the right of the image, is an old trunk hole which could benefit from something planted in it.

Would be greatly appreciated if you could provide me with ideas for these areas. I had the idea of planting a silver or himalayan Birch in trunk hole and maybe putting a Holly where the Acer is? Also thought of blackthorn bush or dogwood where the Acer is? But these are just nebulous ideas and i'm not sure they would work or how many would be needed. There are probably far better options i'm sure.

Many Thanks

 

An Amelanchier would look nice or maybe a dwarf crab apple. To cover the fence I would plant a Trachelospermum Jasminoides.