I was thinking Grey Mosaic tiles around the gas fireplace

The bookshelves and/or cabinets would be out quite a bit from the wall.

slate. it's inexpensive and easy to work with and looks modern and rustic at the same time. then bring the baseboard moulding up and use it and some other mouldings to create a FP surround and a larger mantle shelf. this one is too small. the mantle shelf should continue straight into shelves on the units you plan to add on either side to continue that line. you could use premade shelving units and trim them out with mouldings to make them look like costly custom built units.

plain and simple. that's how we did ours.