This is good food for thought. Yes, working through a niche has been a good "in" for some languages.
Ruby was around for 10 years before Rails released version 1.0. Rails itself didn't get popular until version 2, 4 years later.
It takes a long time for a language to mature to the point to where someone loves it so much they're willing to bet their career on it, as DHH did.
The best thing for a language designer to do IMO is not to try to decide what the killer app is, but rather to make it as nice as possible so that more and more people want to use it to solve problems.
Rails isn't the best web development stack because of any kind of monolithic effort on the part of the Ruby core team. It was built on top of the work of dozens of programmers who did things like write HTTP client libraries, HTML parsing tools, templating languages, all the things that a framework relies completely on.
My advice: You can only focus on so much at once. Let the community build the tools. You build the language. You'll never be able to do both, there just isn't enough time.
There's so much other stuff for you to do. Figure out governance, setting a tone for the community, reaching out to heavyweights so as to raise your profile, working out what your relationship to corporate sponsors is, how you're going to ensure stability across versions, package management. The countless little things that make one language more of a joy to work with than another.
So far I've only been successful with a few shell scripts calling into the tools of the console's SDK doing mass reflection on 75k shader object files among other things. Yet I do half of my home projects in D and absolutely love it.
I think getting D to interopt with more existing code in all of C#, Java and Objective-C as well as on both mobile devices and game consoles will greatly help its adoption with the "oh its just another library we add to our existing codebase" factor which has tremendously helped Clojure take off.
This means D could make it easy to extend both JVM and CLR applications with lighting fast code while getting free access to their respective ecosystems. I for one would love to use D instead of C# to write Unity code.
I don't see the point of compiling D to JVM or CLR byte-code for all the efforts it would involve, but rather the CLR or JVM load native code written in D and have each call into each other easily.
Say I can't reach the performance I want in C# and I want to write that part of the system in D and load it in Unity and then call into native D code from there. Without having to mess with the CLR implementation details. That would definitely allow people to try it out and integrate it gradually until it has completely taken over.