Personally I avoid Map fields in protos. I don't find a huge amount of value for a Map<int, Foo> over a repeated Foo with an int key. That tends to be more flexible over time (since you can use different key fields) and it sidesteps all of the issues about composability of them.

I think required fields are fine, provided that you understand that "required" means "required forever". If you're already using protos this isn't exactly a brand new concept. When you use a field number that field number is assigned forever, you can't reuse it once that field is deprecated. It requires a bit more thoughtful design, and obviously not all fields should be required, but it has value in some places.