Multiple different views and representations at every level - e.g. syntactic (coding to AST nodes, not characters), structural (imagine having editable class/module outline), semantic (editable views optimized for specific abstractions that allow you to e.g. edit anything resembling a state machine in a state-machine-specific view).
Views that give you a vertical slice though code, auto-inlining function calls when you need it, allowing you read and edit a large block of code, and propagating changes to their respective source locations - this is the solution for the "lots of tiny functions vs. few large ones" "clean code" pseudo-problem.
Views that let you control focus. No more constant dealing with exceptions vs. Result<T, E>, 50 shades of async, or how to mix logging into it all - problems to which modern solution seems to be all kinds of monadic bullshit that makes code completely unreadable, unless the language itself gives you some arcane syntax and semantics to hide it all. Instead, have your IDE hide all the things you don't care about - aka. cross-cutting concerns - from your view.
For example, are you working on the business logic, and focusing on what the code is trying to achieve, aka. the golden/success path? Have your IDE hide all error handling for you. Turn all the Result<T, E> return types into just T, making it look as if the code was using exception handling (and doing the handling somewhere else). Then do some vertical-slice auto-inlining to make a specific functionality more apparent. Too noisy with logging code? Turn display of that off. Conversely, if you're interested in error propagation, turn display of all the business code off.
(Think of it as Aspect-Oriented Programming on steroids, in an interactive form.)
This and much, much, more. It starts with a simple idea though: stop thinking in terms of source code as text in files. Start looking at it as semantic units (classes, functions, statements, expressions) in a database of some kind. Instead of opening a file and editing its text, you would query the database to get an abstract code graph, and feed it to a view that renders it the way you need it. "SELECT Foo Bar from Classes, JOIN Fields, JOIN Methods", feed it to an editable outline view. "SELECT" whatever else you care about, feed it through some transformer, to a different custom view. Edit it, and have it automatically apply changes/"refactorings" to affected code.
And yes, editing raw plaintext is something that's often very efficient and we have well-optimized tools for this. But this doesn't mean the plaintext in question has to correspond 1:1 with source code. Instead, you could have the class/module outline view be editable plaintext, so you could regex-replace half of it in 5 seconds, and then press a button, and it would rename and move methods and classes across the codebase, making it conform to your edited outline. Basically what dired mode does to filesystem in Emacs.
Etc.