If any other REPL can get to a point where it can rival LinqPad in functionality then it'll be an absolute winner.

The key features linqpad has that I'd need from any replacement:

* Securely store secrets

Util.GetPassword prompts the user for the password if it's not in it's store. Leverages the windows user credential store.

* Keeps the process running

This is absolutely key, it keeps alive the process even after the script has finished, so you can easily attach a debugger / profiler to it when the script isn't running, then start the script again. It also means you can use AppDomain.CurrentDomain.SetData to cache expensive setup if you're trying to profile something.

* Quickly switch between expression / statements / program mode

It'll intelligently work out if you've written a single expression vs multi-line statements, and you can switch to "program" and it'll wrap what you've done so far in a Main stub.

* Easily define default App.Config files, default namespaces, default nuget packages.

After finally getting a fiddly setup working, there's a "Use as default for new queries" button which is very convenient. Also the "clone query" functionality is similarly useful for copying not just the C# text but all the background namespace declarations, references, config, etc.

And that's about it. I barely use the database connection and exploration side, but I do use it daily for hooking into DLLs to quickly test changes to our internal projects.