The biggest factor is the construction methods; you need to use cheese slopes. Standard slope bricks probably wouldn't work at all. And since cheese slopes only cover a 1x1 area, you need a lot of them. And although lots of sets come with cheese slopes nowadays, it's still pretty expensive to go buy over a hundred of them for any particular mosaic. (A 32x32 mosaic alone requires 1024 cheese slopes... basic math dictates that as the dimensions double, the area... and thus required cheese slopes... quadruple. And where standard brick mosaics allow you to use different sized bricks and/or stacked plates, there's no way to switch out cheese slopes. I suppose you could utilize the 1x2 cheese slope elements, but those are even more rare and thus more expensive. So the required building material is more restrictive with these types of mosaics. That's why I'm aiming at a smaller mosaic size, just so that I don't max out how many cheese slopes I need.
But enough text; time for some concept pictures. Click on the thumbnails for the full sized images. (Since they're too big individually for a blog, and I don't want to upload resized versions, so you can deal with Bricklinks auto-thumbnails.) First image is the combined one, and the second shows the two distinct images side by side.
Hau - Krana Mosaic
This is the mosaic I'm hoping to make at BrickCon. (Yes, not for, at... Steven would get me the pieces when I arrive at the convention and I'd build it up before the public convention and take it apart after closing.) I originally wanted a full Hau and a full Xa Krana, but size limitations ruined the quality, so I shrunk it to a half-mask and half-krana. It might be a little obscure to most of the public, but we could always provide an actual Hau mask and Krana for show. (Also, the krana's going to be lime instead of plain green because there are more lime cheese slopes... but that didn't show up in the original pict.) Anyway, this is designed to be a 32x32 mosaic.
This is a basic change-in-time-of-day image. It's not quite as complicated as the others; mostly the scene is just a recolor, with darker elements showing off the night image and brighter ones showing the day. It would be cool to expand upon the idea; maybe have something going on during the day (beach volleyball?) and then something at night (bonfire?) to add some variety. This one is designed to be a 64x64 mosaic, and I slimmed down the two images a little bit more, because the angled views with expand them out. (Something I didn't quite do with the others.)
Mt. St. Helens Mosaic
Here's another change-over-time idea, and this one shows off some of the major differences. And when I was thinking of what would be a neat change-over-time idea, Mt. St. Helens came to mind. There's a before image, showing the full mountain, and an after image, showing the remains of the volcano after the eruption. So far I think this is one of my neatest designs, because the two images blend in but are still rather different. This one is designed to be a 96x48 mosaic, which might be too wide for a mosaic, but those dimensions best fit with the mountains. This would be a fun one to make, but many would it cost a lot to get those colors.
Anyway, those are three designs I've worked on. I have a few more ideas that would be cool (including some Bionicle ones), but the process of making the two images and then combining them is rather time consuming, so I haven't done more. So I'll save those for later.