If you put a round table and chairs with an umbrella, it will look like you did a round patio for that purpose. Then, put outdoor furniture (loveseat, chairs, etc) in the other areas for several sitting spaces.

Safe fire pit with seating around it? Maybe partial low hedge that hides functional things there by the house....

It depends on your goals. If you want to fix it right, remodel the deck to rectangular, so it doesn't look like there is a bite out of it. Remove the patio tiles/pavers and the edging pieces. Remove the layer of sand (to at least 8") and replace with topsoil. Create a planting plan and then plant foundation plants.

It's not what's gone that makes the spot look odd--it's what is still there--a deck in a semicircle. Change the deck shape/size.

buy a circular hot tub and put it there

If you have young kids, how about using it as a giant sandbox? You could even add a slide on the upper deck! :)

Safety first! That's a big drop-off from the old deck. Please consider putting a railing in that open area.