Recommendations

If an ASIC or application exists, which does what you need, use it.

For applications with low bandwidth, and small computational loads, a mass produced microcontroller is the cheapest solution. Libmpix running on the Zephyr operating system is a good choice. Once they get vector processing integrated it will be a better choice.

The only Open Source FPGAs with MIPI are the Lattice NX17/40 chips. The NX33 adds a USB device interface, but is closed source.

If Lattice NX17/40 is not an option, but you want to use Open Source FPGAs, and you need less than 1.66 Mpixels, use a DVP camera.

If a DVP camera does not meet your needs, consider using a microcontroller with MIPI-CSI in and a parallel DPI or RGB port out. Examples include Raspberry Pi Zero,  and maybe ESP-p4, CH32H417 and Stm-32 N6.  You will need CDC circuits, available as Open Source Verilog, or provided in hard core by GateMate.  You will be limited to 1.66 MPixels.

For applications over 1.66 MPixels, a closed source FPGA is needed.

To easily connect an Open Source FPGA to USB, use EZ-USB FX3 with single-ended wires. FX5 also supports LVDS which can run faster.

For a lower cost USB display, use DVI output to a DVI→ USB Dongle.

For even lower unit cost USB connections, use one of the above mentioned microcontrollers, but the engineering costs will be vastly higher. H.264 or M-JPEG will improve the video performance.

For bandwidths up to 5 Gbits/s and cable lengths up to 15 meters use the MIPI A-Phy standard. While MIPI is proprietary, the A-Phy standard is more open. A rapidly increasing number of products should be available.
For bandwidths up to 5Gbits/s, cable lengths up to 15 meters, and simpler engineering, use one of the protocols not designed by a committee. FPD-link or GMSL. GMSL even allows the control signals to pass via a different side channel.

For displays, the Gowin 1K and GateMate support 640 * 480 DVI @60 FPS. The Lattice ECP5 supports 1920x1080 DVI @ 30FPS. Gowin 20K supports (3840 × 2160s) DVI. FGPAs can drive TFT LCD screens. Otherwise one of the above mentioned microcontrollers can be used to drive DVI.

Military applications should also consider the origin of the components used in their video processing pipeines.