How big an area of soil and how long have you had the tree? Bearing in mind you can't plant closer than a foot to the trunk of the tree as a minimum...

If its perennials you want then many perennial geraniums are happy in some shade. You can get an idea of what's available from Long Acre Plants website as they specialise in shade plants.

phlox don't mind light shade, then there are others eg gilenia, polemonium, [hosta, ajuga lamium of course] at this time of year dicentra, some of the smaller Japanese anemones that don't run.
click on the s at the bottom and look up the shade plant section lots of plants listed there.

I planted violas under mine this year, and they are looking beautiful. I like the idea of using geraniums. I particulary like geranium cantabrigiense, and it tolerates shade well. I think Lamium would work well, and so would ajuga perhaps...eecially a white flowering one to light up the shade. And under an Acer I have cyvclamen coum and snowdrops, which are lovely in late winter. Heucheras might be good, and Hellebores too. Just remember to water and feed religiously or nothing will thrive.