I don't think it needs 'stuff' up there.

I have see plates and porcelain collections placed on top of cabinets.

Personal preference. People who use the area as display have a collection they want to be seen. Most would not purchase items to fill the space. It's on trend to have cabinetry reach the ceiling so spending the money on molding and crown and boxing in the space to give the impression of to the ceiling height cabinetry is also done.

Will suggest that you consider purchasing short cabinets to be added above your exiting cabinets. Those could be used for additional storage or if you make then glass fronted cabinets could be used for display especially if they are lighted. That should solve the grease and dust accumulation problem.

Nothing.

You don't need anything up there, though you might consider lining the tops with waxed paper, to make removing dust and grease a bit easier.

I'm not sure how you'd get the power to it, but a strip of LED lights at the back edges (near the wall) could provide a nice glow at night. That doesn't seem to fit with what I see of your kitchen, though. @Norwood Architects has a similar idea, and a better one, IMO, but also more expensive.

Definitely cabinets above-looks like a good amount of space per the photo. They could be closed cabinets, some glass or some open shelf like the photo below. Remove the cove molding first.

Not sure about the exact size of the space but maybe some cabinets if they can fit would be the right choice since all that space up there is just a huge dust and grime collector A pic of the whole space would also be helpful

show the entire kitchen. maybe we can suggest something else if you think something is lacking.

Nothing is the answer. It was unfortunately a begging and recessed hole created by header of the walk in / flanking pantry, no flush soffit to cabinetry in the original kitchen plan and cabinetry/trim not considered at the outset.

Ignore and vacuum regularly. No plates, no plants. Decorate at counter level, and show the entire kitchen for more help.

Nothing is needed.