There's only a few things that can happen with the MRJ90. The big three can relax the weight restriction on scope or the regionals can convert the orders to the MRJ70 (replacing older CRJ7s). Can the MAGTOW be certified differently (lower) in the US? I can't answer that question and am unsure if they would require new testing. There's absolutely no appetite at mainline for relaxing scope as far as overall additional airplanes or relaxing the 76 seat number. However, I could see them relaxing the MAGTOW number slightly if the carrot was big enough. The other question is how does the scope choke evolve over the next contracts (starting with Delta). Do they allow more 76 seats in exchange for further reducing smaller jets? If they hold on the large RJ 76 seat jet limitation, the smaller MRJ70 may look more attractive. I don't think the MRJ90 is going to be a good fit at any mainline carrier. I think the new E190 or the small CS would work better.
The same questions apply to the E175E2 which on paper is heavier than the MRJ90 I believe.