Can transceiver replace SPI, I2C and other "normal" methods for high speed data transfer between FPGAs? If so, how?
There’s no need to use Ethernet protocol between Intel 10nm FPGA’s.
You can use the differential transceivers with NRZ or 4 level PAM4.
Some devices include Forward Error Correction (FEC) and optional/ configurations:
Transceiver channel count :

4 channels at 116 Gbps (PAM4) / 58 Gbps (NRZ)
16 channels at 32 Gbps (NRZ) /12 channels at 58 Gbps (PAM4) - RS &amp; KP FEC