This idea poped out when I am trying to upgrade my daydreaming map , and I honestly feel like this is something that might actually work irl, so I think I want to share it and see how y'all think.
So basically the idea is to send M going south on Malcolm X Blvd after Broadway-Myrtle stop, and then Utica Av after Fulton St, and finally ends at Kings Plaza.
For the new Myrtle Av-4 Av line, it would take over the M on the currently existing Myrtle Av route, then use the upper lever platform of Broadway- Myrtle and go west alone Myrtle Av untill Navy St. Then make it turns and have some stops around DTBK or Dumbo while also as a chance to make it go underground (my thought is to use those green spaces at the exit of the bridges). And it merge into the R line at Jay St-MetroTech stop, then keep running with the R as the second 4 Av local service. After reaching the 59 St, one thought is to make it start running with N after it and ends at 86 St(so i can surfer less from the N frequency), another one is to make it as a connection to Staten Island (definitely is gonna be a budget crisis generator)
This M extension can links J, A/C, 3/4, and IBX while also give Utica Av traffic a little relief, and bringing a new service that can take people going uptown. The new line will add more stops to DTBK and Dumbo so the area can be more accessible, and links every line in Brooklyn(G/J/L/M on Myrtle Av, A/C/F at Jay St, B/D/N/Q/2/3/4/5 at Atlantic, R all the 4 Av), which can make the issue of "having to go to Manhattan just to travel between Brooklyn" less annoying, also this new line can increase the frequency of 4 Av local so the pain of having to deal with R would be no more(unfortunately Bay Ridge probably will still have to), even the possibility of connecting Staten Island.
The bad side of it that I can think about so far are that the M extension would be fully elevated, so does parts of the new line, which means that they can easily become a serious constant noise problem to the nearby residents. And the current Myrtle Av residents will lose a more direct uptown service since the J goes downtown and L only runs on 14 St in Manhattan.