I’ve had the idea of some kind of “smart” USB add-on card for vintage hardware that you could just plug multiple modern USB devices into, possibly through a USB hub, and then the card would present the connected USB devices to the host system as native legacy devices, negating the need to implement a USB stack for DOS and drivers and emulators and such.
So plug in a USB MIDI adapter, and the card would present it as an MPU-401 MIDI controller to the host systems, preferably with support for Intelligent Mode as well as UART mode. Plug in a USB sound device and the card would present it to the host system as a Sound Blaster compatible sound card. Plug in a USB game controller and the card would present it to the system as a 15-pin PC game port adapter. Plug in a USB mouse and the card would present it to the host PC either as a serial mouse or a bus mouse. Plug in a USB stick or external hard drive, and the system would detect it as an (XT-)IDE device. Wi-Fi dongles would show up as NE2000 network adapters. And so on, and so forth.
I guess the PicoMEM, being a software-defined expansion card, could be made to support all this. A lot of such projects exist for some of these things, but having a single card that could emulate all this, effectively forming a bridge between common USB peripherals and virtual legacy devices that are compatible with hardware that was natively supported back in the DOS era, that would be so incredibly practical.
Of course it might be a challenge for a PicoMEM-like card to support so many hardware IO addresses, multiple IRQ ports and DMA channels simultaneously on a single card with a single RP2040 or RP2350 controller.