Art work in place of mirror panels on windows

All of the above and put the mirror vertically over sideboard :)

Could the chairs change? Mirror could be bigger.. Add crown moulding or wainscot..

Agree on putting the mirror over the sideboard.

Then you have that big wall for some fabulous art.

Can you change out chairs for fabric ones?

A table runner would add softness & color: if you can't find one in the right size or color, they are very easy to make, or find a tablecloth & cut it, or even a drapery valance could work! Recovering chair cushions, if your chairs are cushioned, would add interest too. Your room is really nice; love the rug colors, and windows. Drapes or a flat scalloped valance? Everything in the room is square or rectangular. Adding something round or oval in art work, tabletop or credenza accessories would add interest. If you don't want to purchase 2 lamps, 2 topiaries would look good.

a large original art piece on the wall will gives more life and enhance the look...for more artwork check my shop too...

https://www.etsy.com/shop/PooArtGallery?ref=profile_shopname