1. Entry-level model, maybe in the $500 - $600 range. Something a guy in Mumbai or Nairobi could actually afford. Not everybody in the world needs or wants to have a mobile phone with a stellar-quality screen or a hyper-sophisticated camera. And modern smartphones are getting larger year by year, to an absurd extent. I bet a lot of folks would appreciate the convenience of something smaller that could more easily be slipped in a pocket without making a person feel like he's carrying a Glock.
2. 5g, of course. Until the carriers have an infrastructure in place, which will take years, it won't do anybody outside of major metro areas much good, but everybody's immediately going to want it anyway.
3. The carriers are going to pull the plug on their shrinking, decaying and increasingly unprofitable landline services a lot sooner than you think (how else are they going to pay for that new 5g infrastructure? Otherwise putting it up it would bleed them white -- the shorter 5g wavelength is going to mean they need plenty more towers to cover the same area). At which time the mobile phone is going to need some serious rethinking. The result will probably be a phone with two SIM cards, one with a unique phone nr. and the other with a single number shared by all members of a household or small business. (This means the carriers will have to rethink how they do their billing. I bet they're bright enough to figure out how to do this.)
4. Even with the implementation of conference calling, the conspicuous weak spot of the iPhone is its reliance on that lameass Face Time. Apple needs to replace it with a sophisticated telephony app that can match or surpass such ones as Phone Amego or Dialectic (great features, lousy interface) for the Mac, with features such as call forwarding and call recording. And match it with one for the Mac. A mobile phone should be able to do everything a landline phone can do, and at the moment this simply is not the case. Better software support would probably sell more phones.
5. To my surprise, I recently discovered, almost by accident, that Apple is still selling the iPod Touch, and at a remarkably cheap price (for that matter, you can pick up a used one on eBay for a hundred bucks). As long as it's on the same wi-fi network with an iPhone you can use it to make and receive calls just like an iPhone (using the same phone nr., of course), I've picked up a couple to spot around the house to use just like I do the extensions on my landline system, and when they're not in use they function nicely as desk clocks. If Apple would repurpose the Touch by focusing on that use, and at the same time hold the price at the current level, I bet lots of folks would realize it still has a place in the modern world and its sales figures would come back from the dead. And as far as I know no other smartphone manufacturer offers a similar satellite device. Surely that's an interesting selling point. (NB - only the most recent version of the Touch can be used for this purpose. And rest assured, that model can handle the most recent iOS system and every iPhone app I've tried so far).