"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Ming-Chi Kuo: 2018 iPhones Will Have 'More Complex' Metal Frames for Improved Cellular Signal Transmission
Specifically, oft-reliable Apple analyst Ming-Chi Kuo said he believes iPhones released in 2018 will feature a "more complex" metal frame design and casing assembly for the purpose of improving data transmission quality.
An excerpt of Kuo's research note with KGI Securities, obtained by MacRumors:
Orders for 2H18F iPhones are market focus; both Catcher and Casetek have won new orders. We believe the metal frame of 2H18F iPhone models will be composed of more parts (iPhone X has four parts) for improving data transmission quality. We believe Apple will need more metal frame and casing assembly capacity and new suppliers because of more complex metal frame design and casing assembly, while models equipped with AMOLED and stainless steel metal frame will increase to two (vs. only one — iPhone X — in 2H17). We expect Catcher will obtain stainless steel metal frame and casing assembly orders. If Casetek can pass certification in late 4Q17 or early 1Q18, it also may win aluminum metal frame and casing assembly orders.Kuo said Taiwanese suppliers Catcher Technology and Casetek have won orders from Apple for the new metal frames, including two models with stainless steel frames, which likely includes the iPhone X and so-called iPhone X Plus.
Kuo has previously said that Apple will likely expand Face ID to all iPhone models in 2018, with a complete abandonment of Touch ID. He also said 2018 iPhones are unlikely to gain a rear-facing TrueDepth camera system.