Need your opinion regarding GR8NET capabilities

Haven't seen that video, but this is a cartridge we're talking about? If so, in terms of hardware the additional cost to implement SCC in it, should be pretty low. Low end FPGA's are like what, € 10~15 or so these days? Possibly you'd need some extra parts / IC's to deal with 3.3/5V issues, but those extra parts are the kind of things that cost pennies a piece.

So perhaps the only relevant question is whether you are willing to do such an SCC implementation (probably FPGA-based). In terms of "put in the work" - for whatever reason(s).

If the answer is no, the cost of parts is irrelevant.
If the answer is yes, the cost of parts is -mostly- irrelevant as well.

Nice job, looks “great”! It’d be good to have a network card generally available again (I’d definitely like one). Built-in support for directly loading ROMs from the internet is an interesting feature. I think if the internet ROM loading feature is a primary selling point, then SCC support would probably be desirable.

Thinking about what I’d use that for, perhaps loading Synthesix ROMs directly off my Mac via WiFi? Seems handy for development. For that a built-in SCC would be handy (saves me a slot), but not essential. Can it be plugged into a slotexpander?

I can imagine someone making a nice selection UI to directly tap into a ROM collection made by someone else via the internet (e.g. imagine an MSXDev compo browser), it’d be convenient because you would not have to pre-load all the games on a flash card. If you would offer telnet-like I/O as a Basic device as well (like RS232 BIOS’s "COM:"), it’d be easy for people to make a loader program in Basic to fetch a CSV directory list via HTTP and show the choices in a menu.

I assume it can also be addressed easily from assembly and supports the Ethernet and TCP/IP UNAPI?

I meant something like OPEN "NET:" AS #1 : PRINT #1,"GET / HTTP/1.0" : LINE INPUT #1,A$ : … Well, just an idea.

See here, scroll down for the Ethernet and TCP/IP specifications, if you support that a bunch of tools will be available like internestor lite, hget, ftp, smb, twitter, etc.

With a FPGA, you can integrate SCC, FM, SN76489 (#3F port) and many Rom Mappers support.

The SCC is important for the function of Rom launching because the SCC chip must be in the same slot as the ROM.
The SCC needs a FPGA. The FM (and mostly the SN76489) is not indispensable but if the FPGA can emulate it, I think it is better to integrate it because it does not really increases the price and many MSX have only a single Slot.