Just reading the words "iPhone 13" probably feels insane given how fresh on the market Apple's latest family of iPhone 12s are, but before you commence the eye-rolling, remember that Apple actually designs and plans phones for several years out, meaning whether you want it or not, the next iPhone is already on the way the minute a new one is released.
One of the biggest changes could be the removal of wired charging in one or more models, according to Apple analyst Ming Ch-Kuo. Apple has taken steps in that direction with its line of MagSafe accessories that magnetically snap to cases and the backs of the iPhone 12s (you can imagine what a boon wireless-only would be for MagSafe). Apple has also shown a willingness to do away with offering products that were typically included in the purchase of a phone such as charging adapters and earbuds.
Cutting out the camera notch has also been on the rumor list for some time. Reducing the size of the notch (the cutout at the top of the iPhone that contains the front-facing camera and other sensors) would help Apple increase the size of the display on the iPhone, potentially paving the way for a true all-screen display. Over the last year, somewhat dubious "prototypes" have even surfaced showing what appears to be a notch-less iPhone design.
The iPhone 13 isn't expected to be a major departure from the current design of the iPhone 12 which has flat edges and is encased in a metal frame. Given the amount of time Apple must have spent redesigning the iPhone 12, it'd stand to reason that the next generation of iPhones will stick to that choice for now. The 2021 iPhones are also expected to have the same number of cameras (three) despite some questionable rumors suggesting it could adopt a four-camera system.
Apple has already begun re-implementing Touch ID (which uses fingerprints instead of face recognition to unlock) in the new iPad Air 4, and that could expand with the next iPhone release. The most interesting question is whether Apple will be able to pull off some kind of in-display sensor (like on many Android phones) that embeds a sensor underneath the glass.
Apple will likely continue to work on its new line of ARM-based processors like the M1, which was recently rolled out in its 2020 line of MacBooks and the new Mac Mini. What kind of new features that will enable is unclear, but if the latest MacBooks are any indication, the advantages could be significant. The chip, presumably called the A15 will, like the M1, use Apple's 5-nanometer fabrication technology, which would make it more powerful and power-efficient.