No way would I want a drill or a circular saw to use such a large battery. I've already got a perfectly usable Makita cordless saw, as well as a couple of Makita cordless 18V drills. At some point, battery size such as the Ego's is just too large for certain tools.
Backpack with quick detachable cords would be handy on job sites where you don't have power and need long run time. Would be a niche market but if you could power any number of devices using adapters off the same pack I could see it being useful.