Probably the biggest change I'm aware of is elasticsearch removing index types. We will need to come up with different solutions for what we have previously used those index types for. This can be done prior to moving to elastic 6 though.
Not necessary for initial deployment, but we might be able to remove some hacks we put in where we index keywords as strings using an analyzer that always emits a single token. This prevents us from enabling doc values which would allow some aggregations (for example finding popular categories, etc.)