This is my top one as well. Most intelligence is slow yet can be outsourced to a (on premise) cloud. I find it quite strange that in 2017, there is no universal API to delegate code analysis computation onto distributed nodes. Language servers are a good step, yet this concept should be expanded to (macro) code analysis, automated code reviews, memory leak detection, (unit|integration) tests, large scale refactorings, builds, deployments, basically anything a modern IDE does.
Some of above is already there, but they are separate products which don't integrate well. For example: You can find a distributed build environment for .NET & Java. This also runs test, so there is some level of integration. However, I'd also like workflows like refactor-build-test-analyze-fix-refactor until satisfactory.