Gigabit Ethernet HDMI/DVI PCI/PCI Express External non-serial memory (DDR/Flash etc.) Things that are relatively easy to add, and are not so much of a big deal to wire up yourself.
MMC/SD cards Character (e.g. 16x2) LCDs Anything I2C/SPI and relatively low speed VGA (with low colour depth) I like having a board with many (at least 8) SPST switches and LEDs, and momentary buttons. Unlike a microcontroller where it's relatively easy to spit debug information out of a serial port or to an LCD with a single C function call, debugging FPGA designs is a bit harder. LEDs provide a zero fuss way to break out internal signals for visualisation - if you're tracking the progress of a complex state machine, you can light up an LED when it gets to a certain point without adding any extra logic. While these are easy enough to add yourself, I find that it's better to get a board that has them so that you don't waste valuable user IOs or waste time investigating failures caused by your terrible soldering skills."
Found here: https://joelw.id.au/FPGA/CheapFPGADevelopmentBoards