I don't see how an A500 is less good for everything an A600 can do
The A600 basically has zero "real" expansion ports. The difference is built in harddisk interface. The solution on A500 was to put everything in its real expansion port; harddisk interface, fastram, and CPU. Top of the line expansions had all three in one unit.
I can't really see any ceiling to bringing an A500 to exactly the same level as an A600, regarding performance, peripherals and display. I doubt demand would be lower than for A600, the number of sold units are hugely in favor of A500, and anyone who kept his A600 is the same kind of person who kept his A500.. I bet a lot of people would love to not muck about with side expansions with old drivers that only work on old OSes in favor of a brand new solution!
I think it would have to have a harddisk interface built in, though. That's I think the #1 usability boost, plus that otherwise any accelerator would have to talk to whatever harddisk interface the user has, and even if it works, harddisk performance may suffer due to their design.
ACA630 with a DIL socket would do it for me, if it had an onboard harddisk interface - of any kind.
Maybe if the harddisk interface becomes faster/better than the A600 built in one, these cards could also have the market of the A600 users that never got an ACA630 - they're soon all gone.