This is my thought as well. Support one way (I like errors property in returned json) and then users will have to adjust their APIs to match. Most of these APIs will be custom built to support their dgraph setup anyways.
Another thought is that most of these APIs will probably end up being lambda functions (at least that is my setup) I see that Dgraph also uses AWS to host Slash, it would be neat if eventually Slash supported integrated functions.
Netlify does this with their front end deployments allowing users to build their backend scripts right next to their front end UI. This would make sense in dev writing functions right next to my schema, and could also possibly help generate some more revenue for dgraph SaaS.