I've been turned on to the benefits of the mid drive recently. I've been thinking about a possible build. I would pair the mid drive with a Nuvinci CVT. they're pretty cheap on eBay now.

I've ridden a Nuvinci before and it was too heavy, but I think in an ebike it would make a lot of sense. It would be a clean straight chainline. The nature of the CVT drive would be such that there wouldn't be any danger of it falling between gears. This would be a problem given the power of the electric motor.

I think budgeted out, I could probably build a mid drive system with CVT for roughly...$1300.