Maybe this helps you a little bit further.
If you see what happens on the market then you can regard Firewire as being dead. Its not supported under Win10 anymore, you need to deploy legacy Firewire driver for that and also in the MAC world you see USB and thunderbolt being the new interface standards. Based on that its very unlikely that RME will offer new products based on Firewire.
In terms of USB3. If you see the current RME product portfolio, then you will notice that RME is able to support up to 68 ports IN and OUT via USB (https://www.rme-audio.de/products/madiface_pro.php). RME uses USB3 only in cases, where more channels are needed (MADIface XT 196/198 ch IN/OUT, UFX+ 94/94 ch IN/OUT).
This is for the advantage of the customer based on the assumption, that there are still systems out which have more USB2 resources compared to USB3 port resources and that USB3 ports might be in use already for other purposes like i.e. to connect an external drive at high speed, etc.
So if you talk about a successor of Multiface II, then it would be nowadays - according the amount of channels (<68 IN/OUT) - most likely be an USB2 interface.
The successor of an MF II is IMHO already available in form of an UC or UCX.
Where the UCX would still offer to you USB2 and FW400 and on top useful features like:
- full FX chip on board
- Autoset feature
- Class Compliant feature
- more modern converter compared to UC
If you want to enhance the sound of d/a conversion for phones and speakers this nice compact solution
can nicely be combined with an ADI-2 Pro FS:
https://www.tonstudio-forum.de/blog/ind … our-Setup/
https://www.tonstudio-forum.de/blog/ind … ort-DE-EN/
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13