There is no such killer application for bitcoin. Let's face it, Bitcoin's benefits over fiat are currently not big enough to justify switching to it.
There is no need for a killer application for bitcoin.
In fact, bitcoin itself is the killer application for the internet.
What was email? Just communication.
What was the web? Just media.
What's bitcoin? Money.
What is the most important thing* in the lives of billions of people on this planet?
Communication?
Media?
Money.
* well, there are other things that might be more important than money. Work. Food. Health. But we have yet to find ways to leverage the internet for them.
Yes, money is important. But people
already have money system. They need some very strong reasons to discard it and switch to something else. What benefits this "something else" would give to them? Those benefits should be really big, to overcome tremendous inertia and network effect of the current money system. Email did have such a benefit: it delivered mail
thousand times faster. What thousand-fold benefit over dollar bitcoin has? For average Joe? In an area that really matters to Joe, like speed of mail delivery mattered?