Downsizing an airliner rarely results in a competitive aircraft.
As pointed out above, even an aircraft with the capacity of the B762 is a bit bigger than Boeing's MoM target market, so we'd have to be talking about a pretty drastic reduction in size for the 787 compared to the current/proposed variants of that aircraft.
Yes, it worked (up to a point) for the A300 -> A310, but that involved a completely new wing.
The 737-500 was pretty uncompetitive and sold mostly to airlines operating other 737 variants, where at least it had the advantage of fleet commonality.
But Boeing can't just target its MoM offering at existing 787 operators as that would severely restrict the market.
My money is still on a 2-3-2 twin aisle, for which the 787 fuselage would make no sense at all.