To answer this question I think we need to know what the criteria are to be considered a "major." I would argue, based on the current members and series structure/policies, that it's five things (in rough order of importance):
1. Is it a world-class event (well-organized, best-in-class production value)?
2. Is there a world-class elite field (both men's and women's)?
3. Is it a successful mass-participation event (large field-size)?
4. Is it in a major global city?
5. Does it have a strong commitment to anti-doping?
On those grounds, these are some of the other races I think should definitely be considered:
-Paris
-Amsterdam
I think those are the only ones that aren't currently majors that check all the boxes. Depending on how you weight the categories, others that might be considered (in approximate order) are:
-Frankfurt
-Rotterdam
-Shanghai
-Dubai
-Toronto
-Los Angeles
-Beijing
-Vienna
-Seoul
-Honolulu
-Houston
Others that would be considered if not single-gender:
-Fukuoka (maybe even on the first list right up there with Amsterdam)
-Nagoya
I wonder if, rather than adding marathons, they shouldn't consider adding about 8-10 non-major, international road races (the BAA 10k, the Shamrock Shuffle, the NYC Half, a "Great Run" or two, some major Euro 10k-25k races, etc) and have a second concurrent circuit, the World Road Running Majors, with a similar annual-scoring structure.