Oh well, currently planned production for the MAX is at least for another decade, by then technology will be quite different once again and the 787 will be old technology. And as the proposed savings didn't materialize with the 787s non-bleed design i don't think we gonna see that again. Of course, if there will be a public discussion about the aerotoxic syndrome it might come back for entirely different reasons.
Boeing was studying a clean sheet replacement for the 737 before they were forced by the market to simply re-engine the 737. Indicators were however not for a 150 seat aircraft, rather a 180 to 250 seat one. Probably elliptical fuselage and twin aisle configuration as that allows shorter turnaround times (faster boarding and de-boarding). That is in line with the shifting proportions of ordered 737 variants that focus on the higher seatcount variants with not many for the small ones (only 81 out of over 2400 ordered MAX are -7).