I would dig out a crisp edge to your foundation beds, then plant colorful annuals all along the front with some sort of compost/mulch (2-3" thick) to give it a rich look. I'd stick with just a couple types of annuals so it's not too busy looking. Petunias fill in well and would go nicely with the color of the house. I'd give your edge a gentle swoop rather than straight lines. Play with a garden hose until you get a line you like, then just dig inside the hose. I'd just leave the side yards as they are with the shrubs, but mulch it all well. To me, leaving some 'blank' space would be inviting for a prospective buyer, let them imagine how to fill it in.
I wouldn't put a lot of time/effort into establishing a formal edge, the new folks may want to redo the whole yard (or not). It seems more important to me to keep the lawn cut, the edge crisp, and the beds weeded (the mulch will help that immensely).
Add a planter or two full of blooming flowers to the porch, maybe a wind chime or hanging basket to balance. It looks like a fairly small porch, so be cautious about too much. Clean and simple is better for me, and makes the space seem larger.
I agree that planting a tree and mulching around it in the bare spot would be a nice touch, although getting one big enough for impact may be spendy. You could think about moving the porch bench to the bare spot, site it at an angle, add a tall blooming planter, and mulch all around the balance of the bare spot. Again, I'd just cut in a crisp line and wouldn't bother with any sort of hardscape installation.
Maybe toss in some solar foot lamps, they are inexpensive and add a nice touch.
Good luck with your sale.