Probably they could pull it off to put the base M1 into an AppleTV-sized housing. But I'm not sure it'd be worth it: The internal PSU is one of the major selling points of the current mini design. And the required ports for a "true" computer would need more space than the few ports of an AppleTV.
And it would increase complexity for testing and logistics to have yet another variant, next to the existing chassis for M1Pro/Max (single) and a newly developed, maybe Cube-inspired case about the size of two or three minis stacked for the hypothetic Duo/Quad flavours. That would be a slippery slope, considering the reasons for Apple's near-bankruptcy back in the days before Jobs returned.
I'd prefer them to make better use of the space in the existing housing by e.g. adding 1-2 NVMe slots and perhaps even additional Ram slots, which may be slower than the on-chip solution they have right now, but could be populated by the users themselves. System could then prioritize internal Ram, but would have additional resources when required.
And perhaps make the internals better accessible by having all components on a tray that could be simply slid out. Kind of a modern-day reincarnation of the the Cube's core-on-a-handle approach. DISCLAIMER: Without Jobs at the helm, I think it's highly unlikely for Apple to be brave enough to develop unusual solutions like this.
