Ascender should already exist from WinLemm or Mac sprites. It's just known by a different name in older engines, specifically "Jumper". It's the animation when lemmings step up a height of 3px or more while walking.
I'll also have to put a stop right there on using any assets taken directly from, or extracted using, SuperLemmini - SuperLemmini's licence does not allow for us to do this, and I'd prefer to respect the licence terms of other fanmade engines. If we want to use any graphics, we need to extract them from an official game directly, or create them ourselves. I have in the past successfully been able to extract graphics from WinLemm; not sure if I posted the extraction tool and/or the extracted files anywhere, but I don't remember it being particularly tricky so if I can hunt down the format documentation, I can likely write an extractor again. For Xmas sprites, we would need to either make these ourselves, upscale the existing ones, or find a way to extract them from the Mac version.
I would strongly oppose high-res graphics if they mean any change to existing physics though. Even if they are only an option they should be only a visual change and having 2 sets physics in an engine would be chaos. My main concern was and is always the user created content as that it stays as stable as possible. An engine without functional content is nothing after all and I don't think slightly higher res graphics are worth any sort of physics overhaul and broken level/replays. I rather have 100% functional content in the form of thousands of quality puzzles.
As long as everyone is happy with a requirement of "every graphic set must have a low-res version (high-res version is optional)", I believe I could do this without any physics impact whatsoever. This requirement would make things easier anyway, as I wouldn't necesserially need to have the editor also support high-res mode.