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.
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?
In fact, early email was pretty slow, when you had to deliver over a chain over uucp-relays etc. It could literally take days for an email to reach its recipient. Fax, on the other hand, was pretty much instant. At the same time, fax was not for the average Joe, but email wasn't either.
But, quick email and fax were available to businesses. And boy, did they use it.
Now, about bitcoin, it might not necessarily be available to your average Joe and also not offer him much of a benefit.
But for the business user, bitcoin ist pretty much the fax / mail for money.