Gardening with free trees - serendipity or too much of a good thing?

A Red Pine volunteer from my neighbor's tree is sprouting between my rhubarb and my New Dawn rose. I'm going to move it before it gets any bigger, but where to put it? I finally have some sun in my back yard due to the sad deaths of my neighbor's adolescent American Elm tree, and will have more soon because the other neighbor has two adolescent American Elm trees.

Put it on the north lot line? Make a focal point of it? I'm thinking here could be a great opportunity to, over time, create a little spot of acidic soil where blueberries will thrive.

Other free trees - a whip that I thought was forsythia is actually willow, but I don't know if it is the shrub kind or the big weeping kind. HOw to position it so that, by the time I find out, it's already in a good spot? I would love a living fence including a willow on the north or east boundaries, but if it's a weeper, a specimen spot would be best.

just keep an eye out for the ones you want and transplant the ones you want to keep.
You already seem to know what you want to do.