I do like the River Birches at house corners, and as you mentioned, you might just start over again with one...perhaps pull it out from the corner a bit further if you can.
If space is an issue, you do want to go with a tree that is more narrow than a wider one. Star Magnolia could be a consideration. Another idea is to put a group of three Lilacs.
Here are some other trees you might take a look at: 1. Japanese Red Maple - Acer Palmatum 'Oshi Rini' - one of the more narrow lovely Japanese Maples. 2. Rutgers Dogwood 'Celestial' 3. Gingko biloba 'Fastigiata' - beautiful leaves, about 10 to 15' wide. Gets VERY tall, but it is also a slow grower.
