Ah, progress! Basic support for larger tiles is in now. In fact, I might make the tiles much larger but just display scaled-down versions during the game, which would allow for smooth scaling of the playfield between scales. I used a trick like that fscaling smooth-scaling map screen in Mayflight (a particular favorite effect from that game). I'm not sure I can do this with Pygame however -- one of the reasons I tried moving to pyglet is it can blit images while scaling them in transit. Pygame only scales through the creation of a new surface, which must then be blitted to the destination, which might be too much overhead for me.
One benefit of using larger tiles is the game doesn't have to blit so many images, which is worth another modest framerate increase. The disadvantage, of course, is that less of the world fits on the screen.
In design news, I've started planning out in detail how the random substance properties will work. There are planned to be three kinds of substances: liquids, gases and solids. Liquids are flowing materials like, but not limited to, water. Sand is also a "liquid," for instance. Gases hang in the air and slowly spread out. Both may or may not end up having pressure support, if I can figure out a relatively inexpensive implementation (which is one of the reasons I obsessed over finding ways to speed the game up for a while). The third type, solids, are basically stone walls, which are more reactive in their properties.
So what do I mean by a random substance? The idea is that, at the start of the game, in addition to substances with fairly obvious attributes, there would also be some with unknown properties that must be deduced through observation and experimentation. From the promo video, this is like throwing a torch into a pool of unknown liquid to see if it's flammable. My idea is that the first world the player explores in a campaign would have little or no random substances, but each one after that would have a little more randomness. Another idea, which I mentioned in an interview, is that the planets explored by the player would be divided into solar systems, and each system's would have its own "elements." So, different planets would have different maps and layouts of substances, but the behaviors of those substances would be the same throughout.
If I do this right it could be one of the most interesting aspects of the gameplay. I find that many of my favorite roguelikes are the ones with randomized equipment, which turns figuring out what your stuff does into a logic game. However, it is also true with many of those roguelikes that the identification game isn't as interesting as it could be, or is only a real requirement in the early phases of the game, as once the player learns what everything is there is nothing left to learn.
Anyway, all this is still some time off in the future. I'm going to work on the graphics a bit and fix up the platforming engine before then, I think.