The answers to Does Sails.js or Meteor.js work with ArangoDB or OrientDB? do not contain info specifically about Meteor/Arangodb combination.
This is what I need to know: how close is ArangoDB to state of being a drop-in replacement for Mongo in Meteor?
The reasons I would prefer to use Arango:
Out of the box, Meteor only supports MongoDB directly, so there is no real "drop-in" alternative to MongoDB if you are a bleeding heart Meteor developer.
Because the Meteor server is built on top of Node.js you can simply use the JavaScript driver for ArangoDB to talk to ArangoDB from your server-side Meteor code. Alternatively you can simply use the ArangoDB HTTP API directly.
For a less database-specific solution you can look into Apollo as BennyB pointed out, but keep in mind that Apollo communicates using GraphQL, which while offering a lot of flexibility to frontend development also creates certain limitations for backend development, especially when it comes to optimizing queries for performance. A naive implementation of a GraphQL schema treats the database purely as a key-value storage, which will not play into the strengths you're interested in (specifically transactions won't be available this way).
