Take some close up pictures NOW of the stems, buds, and other plant parts (blossoms) that are evident.
I'm leaning toward a simple leaf species with alternate arrangement, possibly something pendulous. There are Tilia and Ulmus sp. which could fit that description. With growth becoming evident, I'm leaning toward something like Camperdown Elm (Ulmus glabra 'Camperdownii') - though Camperdown Elm is often twiggier, thus the idea of a Linden selection of some sort.
Without better close up details, we're just spit-balling here.