Hi again! Finally I have time to continue with this project! (sorry for the delay) Let's see if I can finish it this time...
Meanwhile I have created some PNG converters for testing the graphics on a real C64. While doing this, I have stumbled over some problems in my old mockups:

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old mockup - new screenshot
If each 2x2 area has the same block color, then I can use a more efficient scrolling code. (I need all CPU time I can get for the game mechanics) Seems like I can rearrange most of the graphics without major loss of detail. In the title pictures, this means that the pyramids in the background lost a color and they had to be moved a bit. And some red blocks in the bottom have to be grey. Not that bad.
If you want to take a look at it on a C64 emulator, here's the converted picture:
compare3.prg
In the green cave rooms, the vines cannot be build out of hires blocks, because the green cave uses a reverse mode. (the background color is brown here, so all hires blocks have to be brown in reverse mode) So I have to build the vines out of wide pixels.

It's difficult to cut off sprites in the playfield if they move to the borders, because the status display uses 8 sprites and there can only be 8 sprites in a row. What about building a new status display that only uses 6 sprites? (Then there are 2 sprites left for game objects in that area, which works fine)
A: old status display (8 sprites)
B: new status display (6 sprites)... boring
C: new status display (3 x-zoomed sprites + 3 normal sprites on top)... meh... I don't know... Any hits for a status display?
All these workarounds could be avoided by tricky programming. (And some C64 games are avoiding these) But sometimes, you have to be a lazy cow to save some time, otherwise you will never finish a project!
to be continued...