Meraki really doesn't like to talk about roadmaps. Being able to use more than two WANs is quite often asked here in the community. But probably the core of the MX has to be changed significantly to make it work. Don't wait for it, likely it won't happen soon. But tell Meraki that you want to have this every time possible.
I wouldn't count on it in the next couple of years. I would love to be surprised, but hardware as old as the MX100 had more than two WAN ports. I have repeatedly asked for more than two active WANs but have always been told that it is not on the roadmap, even though the development team know there is interest in the feature.
Realistically I understand why as I think that the load balancing between the two existing WANs needs to be more controllable, before they consider more than two active WANs.