A number of issues have called for experimentation with reactors:

• Allow side-channel P2P messaging between nodes #476 - a message passing reactor
• mempool cache: use a fast hash #2187, mempool: Change data structure #2147 - alternative mempool designs

There's also issues to allow users to specify certain reactors/functionality not to run

• Allow nodes to run without mempool #2233 - run without mempool (like setting size to 0)
• Ability to run seed node without an app #1686 - run without the app
• Tendermint "modes" #2237 - tendermint modes
• p2p: Check peers have certain channels #1148 - check peer channels

And of course we're still looking to add a state-syncing reactor: #828

Taken together they suggest the need for better interfaces to composing the set of reactors and substructures that constitute the node.

As suggested in #476 (comment), we should consider refactors to the node package that simplify extending tendermint nodes with custom functionality without forking the repo or copying all of node/node.go.

Copying node.go isn't the worst as is, but we can probably do much better to minimize what needs to be copied.