This unlocks what is for me the most appealing computing fantasy: I have exactly one device to compute with. At home or work, I plug it into my dock, where it pairs to an eGPU/monitor, external storage, keyboard, and mouse, and is my desktop computer. There, I can access the 'pro' level apps we're discussing below.
On the couch or on the go, maybe I slide it into a bigger touchscreen with some extra dumb batteries, and it's now a tablet. Throw in a keyboard case, and now it's a laptop.
And if I'm sitting on the bus or walking to work, it's my phone, and I can edit today's lecture right there, or access all my files. Maybe I'm not firing up emacs or MATLAB, but it's still there if I needed to.
This, to me, is the fantasy of convergence. And considering we're now currently paying $4000+ for the trio of a laptop, tablet, and smartphone, I suspect one could create something pretty compelling and high-end and still feel like a bargain to many.
Back before Thunderbolt, I used a mini-PCI-express to PCIE adapter for more GPU power. This was great for games, but I immediately became CPU-bound, which wasn't so great.
To fix this, you could have something like Big.Little but where the Big cores are external and only used when connected. Heck, then you could do something like a Big.Medium.Little architecture, where the phone's integrated Big.Little bows to the higher IPC cores in the eCPU device.
In this way, you would be able to have something that was a "soul" device that had all your settings and stuff on it, with expandable CPU & GPU computing resources. Expanded storage would also be pretty neat, but then you'd have to figure out what did and didn't need access when you were away from your "exosuit".