Hmm something like eGPU, but for "real" CPUs, would be very nice... and interesting. For example, using a low-power ARM core for "daily" work on a laptop (e.g. browsing, text processing), and engaging a heavy-duty x86 CPU only when needed, e.g. by x86-only software...

most may not, but many do. There are also many workloads that require a discrete GPU running at medium to high load constantly. Granted I have an older iPhone, but my battery life drops from ~1 day of normal usage to ~1hr of intensive usage, and generates an _incredible_ amount of heat.