That's a very nice thread! I am eager to read about any good approaches as well.
I think that Rygar is a nightmare, because of the colourful double playfield scrolling.
When thinking about OCS/ECS hardware you only have the possibility to use 3 planes for each playfield, which gives you as little as 7 colours for each scrolling plane and allocates all available DMA slots in the display region. I never considered double playfield mode for any of my projects because of that. It usually looks ugly, when you don't have a very talented graphician.
The other possibility would be to scroll the background plane normally, and redraw all foreground tiles over it, in each frame. But there will be a lot of them, so it is impossible to achieve 50fps.
Other tricks, like using sprites, seem also impossible for such a big and colourful plane.
I would either drop the background plane completely, and replace it by copper bars and some sprite tricks, or accept to run the game at 25fps (or lower).
Maybe copper and sprites wouldn't be too bad, as there are often repeating bands of water, trees and mountains in the background. You probably wouldn't notice too much, when repeating the same sprite there.
I would try to go for an OCS system with 512K Chip + 512K Other memory and I usually prefer double buffering.
Sprites can be used for the background plane. Otherwise for the main character or the shots and explosions.
The small screen width of 256 could be another problem. It looks stupid to have it the same width on an Amiga (although I have seen that in the past). Maybe I would extend the scrolling area a bit (but not so much that I would lose any Sprite DMA).