For a classic/straightforward programming interface,
the Massbus disks (RP04 and successors) are a good choice. That will take you just over
500 MB, if you emulate the layout of the RP07.
Current thinking (at least my current thinking) is RK11 first then
probably RP11, both optionally extended to support Q22 addresses. Also
something we're calling the RQ11 which will be our "native" interface
with variable sized disks with a 32-bit linear block address giving 2TB
disks for those who are willing and able to write their own device
drivers. Finally, most likely the RH11 for some Massbus disks with
22-bit addressing. After that, I'm thinking to call it good and move on
to other projects though I'm certainly willing to talk to anyone who has
a particular disk controller they want to implement.