You can plant some Winterberry and some Viburnums.you can add some hostas and clematis and bulbs...
Important is how you match them.you need a designing.
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Thank you designonline for the ideas. I never considered clematis. However, I'm looking for something quite tall for the middle to fill the void. And I'm not sure if it belongs in the very back of the border or pulled out 10 feet in front of the maples. What do you think about a weeping alaskan cypress?

Designing one chunk of a yard in isolation is always a bit of a crapshoot for people on a forum - we can't see the rest and so can't match it or avoid repeating it or being totally disconnected from it.

Also, when there is no driving purpose to an area, at least as far as we know, and you've even given very few details about what kind of a growing area it is (where is south??) then the planting design is almost totally a matter of personal preference. Strictly speaking, no plantings at all are needed here, so without an objective other than to make it "look nice," I don't think there is much an outsider can offer.

What makes you think you need something tall? If you do plant something tall, you will need to keep it growing upright as it may try to snake out from under the canopy towards the light.

At this point you have actually quite a nice view of a neighbour's tree framed by your two trees. The weeping alaskan cypress would block that, without providing much appeal of its own. Or is the idea to block views from those neighbours?

You need a purpose for landscaping. Once you establish that, the question of plant selection may answer itself. The growing area - did you say these are maples - may also dictate some things, for example, there will be root competition from the trees pretty soon, if it is not a problem already.

Karin L