In time, it should fill out behind the lilacs if there is enough light coming in there. I believe the persians tend towards more of a "mounded" form as opposed to the more upright S. vulgaris, so there's a better chance for them going wider than tall and narrow. If you did decide to plant something behind, it would probably need to get quite tall (eg., something columnar like various arborvitae, etc) so as not to be shaded out. Alternately if the wall next to that area were solid (no windows), then perhaps a climbing rose on a trellis could be put there.

With respect to pruning - they can bush out when pruned, however make sure you do this just after bloom in the spring so you don't cut off your unopened blooms (the persians tend to bloom later than the common lilacs IIRC). Also don't wait too late as these form buds for the next year's bloom during the summer. They will tend to form 1 - 2 branch shoots at a leaf node below a cut.

Some examples of low-growing supposedly deer resistant perennial plants that could take over after the spring blooming bulbs and peonies include hardy geraniums and dianthus. You could also try some clumps of sedum for late summer/fall bloom.

Don't plant anything behind the liacs. You want about two feet between plant material and the house so you can get to the house and to allow for airflow. If the lilacs get wider than they are now, you won't have that space.

In PA you also might have snow or ice coming off the roof and landing on plants. Another reason to plant out a bit from the house.

Thank you for the replies!