WalkingTour

So, since it seems like it will be a long time till we could actually meet up, I made a little walking tour for you.  Just follow the redstone trail from point to point and read along.  Please just remember that these are my thoughts alone.
The tour starts at X: -641, Y: 151, Z: 246

First, a general statement about shapes.  Don't build squares.  If you're at Point A, you can see I've built some weird plus shape out of rectangle shapes.  In Minecraft, everything is made of squares, so if you make square buildings, it will be less interesting.  Even with my plus building, I have little round bits (you can see one in the left corner if you're looking at the Point A sign) to further make it less square.  This is also part of the appeal of the sky island for me.  It's not square and thus more interesting.  When you're inside the structure, you can use squares, but squares for the building exterior are not fun.

Point A: Walls
I have a belief that your walls should be made with three different source blocks/textures.  If you look, you can see that I have used Diorite, Quartz, and Oak Logs.  Even though I am using multiple different types of diorite (stairs, slabs, fence, and blocks) because they have the same texture, it does not bother they eye.  The logs are used to differentiate between the different floors, otherwise it gets kind of confusing for the eye.  The diorite columns give an impression that support columns are actually used, which people like since it makes the structure look more believable.
Glass does not count against your texture limit.  Glass is just air you can't walk through.
Advanced point: if I had used a lighter-colored material, like birch planks, instead of oak logs, I could add a fourth, darker texture that would immediately draw the eye to it since all the other textures would be light in color.  You have to be very careful when doing that, though, since it will draw the eye and be under increased (even if unconscious) scrutiny.  This is sort of dangerous and will take multiple experiments to get right.
Regardless, don't settle on your walls.  They shape the rest of your structure.  I rebuilt these walls four times entirely before I found a size and shape I liked.  (Also, those arches went through many redos and are the sole reason my floors look like they have four blocks of airspace rather than the somewhat accepted three.)
Also, you'll want to make a general outline of the first floor on the ground so you get a grasp on the structure's size for later.
Now, if you'll follow the redstone trail, you'll end up at Point B.

Point B: The Corner.
This just proves that you will have errors that you just have to ponder.  If you look up, I have removed a diorite block just to show that the logs come together in an awful fashion.  To solve this problem, I basically just stared at the Blocks page on the MC wiki until I saw the diorite fence and got inspired.  Part of building is identifying problems and just trying to figure out how to fix them.  Don't worry about walking away from problems like Point B and coming back to it later, but just remember that you might have to readjust the plan slightly to fix the problem.  For example, those columns to look right in my mind required me to expand the length of the wall by a block.  This was the cause of one of the wall tear downs.
Now please proceed to Point C.

Point C: Torches are ugly.
Here we can see that torches are ugly, but I'm sure you didn't need to be told that.  If you have big rooms, like the one you're currently standing in, torches are especially bad.
Alternative exist, like the glowstone block in the floor, or you could use a redstone lamp if that's your thing (just remember the lamps require power).  Lanterns can be used to hang from objects on walls or put on tables.  They can't "hang" directly on the wall and must have an object either above or below them.  Or you could just avoid the problem by not having really large rooms so that only a couple of lamps suffice.  This is something you will have to take into account for your interior floor plan.
There is a sneaky way to make the redstone lamps work very well, and we'll come to that later in the tour.
Now please proceed to Point D.

Point D: So many problems.
Now this point has so many problems.  The floor is monotonous, even if it has a nice, tile look.  There's a big empty blank space on the wall.  More arches, which threaten to increase monotony.
First, you can see that these arches actually are different than the exterior ones, so it's less monotonous, even if they are next to each other.  Both arch designs take up the same amount of space, oddly enough.
For the wall, art is the obvious solution, but banners can work too.  You'd just have to play with the space to see what works for you.  The arches could be moved inward a little to make the middle space smaller, but then you'd have to worry about the outer small spaces, which are small enough to be fine right now.
For the floor, you could put the glowstone blocks in it to break up the monotony, like I have in the arches.  You can also put carpet on the floor, as rugs on floor can look pretty nice if you go through the effort to design them well.
Regardless, I have left this pretty plain so you can imagine solutions for yourself.  I already have plans for the space.
Now, to Point E.

Point E: Space and ceilings.
Now, if I used polished andecite for the floor on every floor, that means the ceiling would be polished andecite too, which is also monotonous.  (Please ignore the dirt, it's for the next Point's point.)  Because the story is one block higher than normal, this means I can basically decorate the ceiling to not be cold stone.  For example, I've covered the corner of the ceiling here in a few wool blocks.  It's obviously not a final design, but it's an example of what you can do.
Now, please proceed up the scaffolding to the final point.

Point F: The return of lighting problems, lighting solutions, and cheap floors.
Not everyone knows this, but carpet can be placed on torches.  The redstone torch, in this case, provides the power to the redstone lamp, giving the floor a stylish light, and no one can see it because of the carpet.  Also, because I have decorated the ceiling of the previous floor and am using carpet here, rather than using a more "expensive" block, I can literally just use dirt for the floor underneath the rugs.
There's no sign for Point G since I haven't finished building, but...

Point G: Other exterior points.
To break up the exterior boxiness, you could use balconies on non-ground floors.
Stairs lead to a very vertical and sloped roof.  Slabs give a more gradual slope.  Don't make a flat roof.  Even if you make a fairly square building, you can remove that squareness by adding a round tower-like structure on a corner, whether that be an actual tower or a a sky-viewing platform (sadly without a telescope).  You don't have to use circles, though, as you can see with my plus-shaped building.  Go crazy with hexagons or octagons.
If it's a big structure like my castle here, not every room needs to be one story tall.  A grand ballroom that's two or three stories tall with a grand staircase that goes to upper floor rooms is fun.  This castle, for instance, is going to have a large fountain in that center area, with the arms of the plus having normal-sized floors.
Most of all, you just have to get creative and accept that you're not going to perfectly replicate a real world structure.  If you can get most of the way there, though, people's imagination will make up the difference.  Don't sweat all the details too much.