or just make the steam deck your primary hardware platform and ensure it can connect to everything and use all peripherals. refine it. make it unbeatable.
I could see the new Steam Machine essentially just being a Steam Deck in a box. That'd allow it to have beefier hardware but it could use the same software and interface. Add a new tab for HTPC services and a quicker way to get to desktop mode and you're done. It would be another hardware platform but there'd be a lot less design if they were similar in architecture.