Expandable with the three possible expansions available for it, SRAM, card adapter, and Ethernet. That limits it, and you'll have to choose one and only one. Forget fastram if you need a transfer card, and forget internet if you don't have an accelerator (same as for A500).
Well, you can swap them out, but if you have networking, you don't really need the card adapter.
In any event, its still stuff the 500 doesn't have. Most people aren't keen on null modem cables anymore for file transfers.
Yes, for any accel to sell for A500s it would need a harddisk interface. 68020 accel without harddisk interface... no point in bringing out the A500 from the closet, there will be endless support tickets if misc old harddisk interfaces are used.
An 020 card with some fast ram, and an IDE interface would be pretty nice as a boost to the old girl.
I doubt A600 sales came close to A500+ sales, and it has ECS and kick 2.0. So.
Not that ECS/2.0 ever played nice with anything... 
Yeah, ECS is kind of the bastard child

Wasn't the 500+ only out for some 6 months, before the 600 replaced it?
I agree though, that there's undoubtedly a market for a decent 500 expansion. My thing is, that I can understand why they didn't do that one first.