Intel has published a proposal for a new legacy-reduced architecture called x86S (PDF). This architecture is intended to simplify systems that for long have had no need for real-mode execution or any other such thing.
This project is my attempt to run through what a minimial boot and kernel process looks like in the new architecture.
3.3 Removal of 16-Bit and 32-Bit Protected Mode
16-bit and 32-bit protected mode are not supported anymore and cannot be entered. The CPU always operates in long mode.
3.11 64-Bit Reset
The CPU starts executing in 64-bit paged mode with a 4-level page table after reset. The fixed reset RIP is the standard reset vector 0xFFFFFFF0 but is entered as 64-bit. The fixed reset CR3 value is 0xFFFFE000.