In 1 setence predict the next bitcoin killer app

Bitcoin IS the killer app.

I know you said one sentence so there it is, but for an explanation read on. And I know there are other virtual currencies which may one day supplant Bitcoin but for now, I think it is safe to say that Bitcoin is the leader.

There are already very good reasons for the use of Bitcoin:
1) This is by far the biggest: ANONYMITY - Over the past 15 or so years, we have taken our anonymity for granted. The general population doesn't seem to care who knows what about us or what they might do with that information. The Snowden incident and lack of public backlash proves that. Today, with so many hacks happening on a regular basis, e.g. Target, Nieman-Marcus, Home Depot. eBay, Michaels, etc., etc., etc., being able to pay for something on the Internet and remain relatively anonymous, or at least our payment information anonymous, is hugely important. No one is safe anymore and people in general are just on the cusp of figuring this out. It is only a small matter of time before people start clamoring for safety and security with their money. Apps like Isis Mobile Wallet, Google Wallet, Apple Pay, etc., are popping up to address this but they still have the problem of centralization. They can still be hacked. The primary aspect of Bitcoin that sets it apart from anything else is DECENTRALIZATION. No person or corporation or bank has to know anything about you to be able to use it.
2) Merchant fees - People take for granted or generally are ignorant of the fact that merchants pay A LOT of money to payment processors, anywhere from 1% to as much as 5% per transaction. Think about that. If your profit margin is 10% and you have to give away half of that just so someone can use his card, that is HALF of your profit. You could nearly double your income as a business owner by not having to pay these fees. Most people who own credit cards think that the card companies make their money by charging the cardholder interest on his account. But this represents only a fraction of these companies' income. It is the merchant fees that most people don't even know about that are the boon.
It was made unlawful many years ago, due to heavy lobbying by the payment processors, for a merchant to charge an additional fee for using a credit card. I know you may have seen a little sign at some little convenient store run by some poor immigrant from another country trying to make a living which says you will pay a fee for using a credit card. You may have seen it but it is illegal. HOWEVER, it is not illegal to give a discount for using other forms of payment such as cash or Bitcoin. If a merchant started offering a 3% discount for Bitcoin users, do you think people would notice? And he'd still be coming out money ahead.
3) Fraud protection - Four times this year alone I have been notified by my bank that my ATM card was being deactivated due to fraudulent activity. My card has always been in my possession but my information is at the mercy of any business that takes my card information. Each time, the criminal used my card at various places. Sure, my bank was nice enough to remove those charges but it took me being on the phone for 30 minutes and then having to wait days until that money was returned. For some people, having to wait 5 days for $200 to be returned to them would hurt severly.
Not only that, but those charges that were reversed...who paid for them? Certainly not the bank and not the card companies. The merchant got SCREWED because he took the card from a criminal and it is his fault for not verifying information. Merchants are screwed out of payment on a regular basis and there isn't much they can do about it. As stated above, a merchant is not allowed to charge a surcharge for taking your card. They have a choice of accepting credit cards or not but do they really have a choice? How much business would they lose if they accepted cash only?
And not only that, but it could take days for that merchant to actually be paid by the payment processor, whereas Bitcoin payment is received in a matter of minutes (up to an hour to be for sure, for sure). This may seem like a long time for an in person transaction but it is nothing for the online business. But once that money has been received, there isn't anyone who can take it from them, unlike payment processors reversing charges for fraudulent use.
4) Send money anywhere in the world in minutes
5) Be your own bank and pay no banking fees
6) Other illicit activities. There are a number of illicit activies that, although illegal, would be of benefit to some. Such as: money laundering, financing terrorism, ransom and extortion, Silk Road, etc.

So Bitcoin IS the killer app, but I understand what you're asking. If you look back at the PC, it was the spreadsheet (Visicalc) that was the killer app and made every business owner want one. But it took ease of use to really make it mainstream. Apple brought out the Macintosh with its GUI and point/click interface. Microsoft soon followed suit with Windows and then the PC market started to really grow. Then with the Internet, which had been around LONG before 1993, it was the Mosaic web browser that got the Internet as we know it going. Then AOL sent out all of those CDs which made it easy to connect to the Internet. Then broadband started getting implemented everywhere making the dreaded modem a thing of the past. It was the ease of use that got people on it.

It is not a killer app for Bitcoin that is needed at this point. What it needs is simplification. Dumb users need to be able to use their wallets without fear of being hacked, or inadvertantly sending payment to the wrong address, or sending the wrong amount (which they'll never get back). There needs to be easier ways of acquiring and selling Bitcoin for both end users and merchants. Volatility is sort of an issue but if a user could quickly and easily convert dollars to Bitcoin and make his payment on the spot, or vice-versa, then volatility kind of becomes moot.

It is also item #6 above that is slowing the acceptance of Bitcoin. These items need to be addressed in a way that satisfies the general population and the government agencies who feel threatened by Bitcoin.

When these types of things are developed, you will see Bitcoin take off because it really is the killer app.