i believe you can get muntins on casements

There are likely different grille patterns that could be used for a casement but it would need to complement the architecture -it's hard to make suggestions without seeing the house

The window you have whited out seems much more appropriate with the rest of the detailing shown than a casement. Showing the rest of the house would help since all the windows should be cohesive

That would essentially be a 3' x 6' casement. Readily available fro most manufacturers. I would be certain and use a quality brand for a unit that size however.

Or you could do a stationary panel over a smaller casement.

I need to see the house post pics here in comments DO NOT start another post BTW I dis;like grids in windows and without see ing the house who knows.

Recent thread showing the house: https://www.houzz.com/discussions/6414226/casement-vs-double-hung-vs-mixture#n=16

What's behind that window? Maybe 2 windows in an enlarged opening instead one big one?