Instead of running the narrower subway all the way to the floor, have you considered a wider ("taller") tile for the first course, to simply act as a baseboard or transitional piece from floor-to-field? Doesn't have to be an actual baseboard type of tile, or even a contrasting tile. The tile can be the same color as the subway should you prefer it to blend. It can be a simply 6" or 8" square field tile from the same manufacturer. As you know, blending colors may help with hiding a discrepancy, contrasting colors can accentuate it. On the flip side, you could separate the base from the field with a liner or some other detail that melds with details you have higher on the wall, at the chair rail for instance. Again, if the liner contrasts, it may accentuate. A basic 6" square tile, or to keep the subway shape, a 6" tall rectangle, would absorb the floor slope better than a "shorter" tile like your current subways. A 6" tile going from 5" to 6" would hide the discrepancy better than a small subway field tile going from 2" to 3". An 8" tile from 7" to 8" might be even better. If you were to use a true base cove tile like a sanitary cove base, use a basic sanitary instead of a shouldered sanitary, as the shoulder will be cut off due to the slope and that could create lippage or padding out issues as you go down the wall. Sometimes I've even used a taller painted wood baseboard to transition from a sloped floor to the field tile on the wall. An 8" tall wood baseboard scribed from say, 7" to 8" to hide the floor slope, with cap and base detailing, would pretty much hide any height discrepancy from the slope/scribing. The easiest and least expensive "all tile" solution would probably be a basic 6" square tile, scribed and cut from 6" to 5" as needed.

Suzieque ~ I have a little more time this morning to answer your questions. The tile type is mosaic, it comes in sheets that are connected. The challenge is lining it up just right o that you cannot tell that they came connected in sheets (no 'seams'). Our sheets were mounted on mesh. Another type is connected by rubberish 'dots'. The mesh worked well for us to gently pry out the white tiles we wished to replace with black. However, I believe the tile with the dots is easier to lay properly because there is no chance that excess mesh at the edges will bump into the next sheet as you place them into the thinset. To eliminate this possibility, I spent some time trimming the excess mesh from the edges of my sheets. You can buy hexagon tile already in a couple of patterns from daltile. You can also buy octogan and dot tile fairly easily at most tile stores, where you can have a choice of various dot colors. Its not difficult to create your own pattern, but be ready to be very careful and meticulous in your planning. As I said, I trimmed every mesh sheet, laid them out dry-fit on the bathroom floor, laid the black flowers on top and figured out exactly where they would go marking with a pencil. Then removed each of the white 'flower' tiles and placed the black 'flower' (which I had cut out still connected with mesh holding the 'flower' together) They are not glued, until the whole sheet is placed into the thinset. Since our bathroom was narrow, I was able to make four piles of tile sheets (with the black flowers in place) which were in the order of which they would be laid with the furthest row by the window on top. The jagged edges around the room were covered up by wall base tiles, door trim, or the radiators. By the shower pan litle 1/2 tiles were put in place (these can be bought separately, but a good tile setter can cut these). Lastly, I bought our tile online from American Universal tile. It comes in several sizes and finishes and many colors. We chose 1" matte finish. Send for samples first in the various sizes and finishes. You could also be able to order the daltile or american olean from tile stores. Here is a link that might be useful: Source for our Hexagon Tile

To start, a small narrative on what you don't like for your current bath and what you are trying to achieve.

@Angela - The OP's first paragraph stated marble over a concrete slab. " I really think you should remove the old tile." Why? If it's set properly It should last the lifetime of the structure. Bonding to it is not a issue. You'd need to remove the underlayment/top layer of subfloor to even the floor anyway after you rip out tile. If this was wood framed how could you determine that without knowing what underlayment was used and condition after rip out? Plywood, mud bed, CBU's, uncoupler, antifracture membrane etc. The OP is trying to gain height. Most likely you can leave the underlayment in and apply a self leveling underlayment on the top to achieve desired height.