Hashicorp has release a preview of the up coming Terraform 0.12 release. The announcement post covers a number of topics; from improvements to the HCL language,
My favorite updates:
First-class expressions, no more wrapping expressions in quotes.
Reliable JSON syntax, Terraform 0.12 HCL will have a 1:1 mapping to JSON key:value syntax.
The 0.12 is a breaking change release, though not a major one with a upgrade guide in the works.