I'm a bit obsessed with standards at the mo, so I wold put in some type of standard with lots of lovely frilly trailers underneath it. Tumbelina petunias maybe. I have a full whisky barrel that I couldn't bear to half, so I have a large shallow planter on top, with a small water feature in the bowl, planted around with surfinias this yr, but the tumbelinas looked better last yr. Will look for a pic of this yrs, but I know I have none from last yr as computer died with all my pics. (and who hadn't backed them up, daft cow?)
Needs a specimen of some type, then you coild see plant and tub in all their glory in the 'bald' months, underplanted with shortish bulbs for spring, then the traily things in summer. Trailing violas might look nice in late winter/spring.
I had never seen or heard of a tree dahlia before (have a tree peony, but have not had a flower yet in 3 yrs!), so I looked it up. Only glanced at these links so far, but thought you might like.