Firstly, I must say I appreciate your hard work!

I'll take a look at this when I have time, as it changes quite a lot of files.

I know this was done a while ago but I like the idea of it a lot.

(I checked out the PR and obviously, the master branch has moved on since then so I wasn't trying to rebase it.)

I agree with the sentiment completely. It would remove all of the sync and async duplication.

My initial thoughts were that instead of needing EvenCallback, we could simply wrap all sync actions like so:
(plus the argument versions obviously)

public static class ActionExtensions
{
 public static Task ToAsync(this Action action)
 {
 Task SyncWrapper(Action sync)
 {
 sync();
 return Task.CompletedTask;
 }
 return SyncWrapper(action);
 }
}

and then you would treat everything as async internally.
I believe the only thing to deal with then is someone calling Fire() instead of Fire.Async but I'm not sure a consumer would define async delegates but then call the sync version of Fire. But I think this can just use a Task .Wait().

The complication I hadn't considered was how EvenCallback deals with old skool Tasks and returns TaskResult. I need to get my head around supporting the older versions of .net pre the async await days.

I would be happy to give this another go though.