HighGFX also works on ECS chipset. Not all that colourful, but still better than the standard screenmodes.
Indivision ECS uses a totally different concept - again. It's not taking the picture data from the Denise video output, but it's taking the data that comes through the DMA channels from Agnus. In theory, we'd have the freedom to invent totally new screenmodes, but that's rather pointless with little to no software for such a toy.
We might play a little with a custom mouse pointer, but that's all I'd go for. I really don't want to create a new platform.
Jens