Je répondais à @M.L. et @samuelmath.
Yes, a light metro is nothing like an archaic, diesel-pulled, heavy rail train. It comes down to planning, and if an eventual promoter of a rapid transit system in the east can bring transit to the users, and not the other way around. As both posters pointed out, the ND alignment offers a ready-made corridor but poor access and generally low density to the east of Georges-V. The questions remain if density can be developed, or if feeder buses or other local transit may help with ridership spreading across all modes.
The Train de l’est has flaws that can’t easily be corrected, even if it gets converted to rapid transit standards; don’t get me wrong, I’d love to see that attempt happen before any rapid transit is sent out to the second rings or beyond. It’s very easy to point out similar flaws in REM-1 but that doesn’t mean they should be repeated a third time. The Caisse will take part if they see potential ROI — whether in rider fees or real estate development. In REM-1 they obviously saw both possibilities because of all the gifts they managed to negotiate to their benefit. It remains to be seen whether such potential exists for a possible third-party promoter of REM-2, or whether it becomes social service like more normal transit development.