Fill it in with concrete or put gray pebbles and put flowers in pots.

Fill it in with a light grey paver, close in color to the concrete. Doesn't need to be an exact match. It would be a decorate detail.

Its going to be a bare, hot patio without some plants. Move some of the seating so folks can sit around the pool. I like Doug's idea of slender trees, but you don't want to be skimming for all those Crepe Myrtle blooms as they fall.

Pavers.

Poured concrete in a contrasting color.

lambs ear and pansies and fill with gravel

I would LOVE to know what you did we have the same problem

Tall dominating house facade needs a tree in that bed to soften its visual impact. And as mentioned once it sized up enough a tree there would provide some shade for the patio.

Concrete it over and add an island for a bar.

A raised planter could also do double duty as a place to sit. I don't know where you live but, I'd plant ornamental grasses (e.g., purple fountain grass, mexican feather grass, pink muhly). They thrive in the heat, are low maintenance, and require little water.

Seems an elevated planting of grasses in flower could be a problem for any hay fever suffers trying to use the area. Helictotrichon sempervirens for instance is quite a trigger for me if I get close to the inflorescences when pollen is present.

If allergies are a problem, then perhaps aloe or yucca, if climate allows. Both, tough and low water needs.

Before you fill in the space consider whether you would like a shaded area near the pool and whether you have an opportunity here to sink supports for a structure.

Then find out from a pro what you can do to safely a durably bring it level with the rest of the concrete. If it can be done then draw up a layout of how you want to arrange seating for eating and relaxing or shade. Either your new seating arrangements will cover up the patched over area to such an extent any mismatch is trivial or you‘ll want to think about how to make lemonade out of a lemon and iuse the patched over area as one element of larger decorative design on the concrete that you create. There are a zillion sites on the web for creating designs on concret.

Reading the comments it's amazing how people did not HEAR what the OP's end NEED was; expanded seating. Pavers or concrete with some sort of decorative detail were the only viable option w/o tearing it all up and starting over.