I suppose it depends on one's interests.

For example one could try to clone many of these: https://en.wikipedia.org/wiki/List_of_7400-series_integrated...

Another fun project idea. Make a 1bit processor slice. Then show how many slices can make a larger system. Similar to:

"Using LSI processor bit-slices to build a PDP-11—A case study in microcomputer design" https://dl.acm.org/doi/pdf/10.1145/1499402.1499444

A full 6502 is only like 900 gates. One could do a neat project with separate ALUs chips, control logic, etc, like AM2900-style.

From Googling, it sounds like the 74181 ALU chip was only ~75 gates or so https://en.wikipedia.org/wiki/74181. And the AM2901 something like ~500

"I could make a motor driver!" was the first thing that came to mind. I have a need for lots of motor driver chips for an art project. (Think hundreds or thousands of small DC motors.) At a minimum, a basic "h-bridge" circuit to control one motor is 4 transistors (and possibly 4 additional diodes). I'll still have to figure out if TinyTapeout could handle the voltage and current of one small DV motor, though. (5-12V, possibly up to 800mA). Of course, there are simple h-bridge chips already on the market for $0.20/chip, but designing my own would be fun.

Maybe an idea: Could I use this to put a secret key in the gates, with some mechanism to prevent brute-force breaking? So my own personal Yubikey essentially.

Not sure how to protect against replay, with only 200 gates...

It mentions a possible set of dip switches and a 7 segment LED. You could make a puzzle game of "lights out" where only a certain combo of switches would have all the segments off.

Hashing?