What if 2^32 is just not enough?

Don't you think a BIGINT UNSIGNED would be sufficient? That's a range of 0 - 18.446.744.073.709.551.615, or one year with 50.539.024.859.478.223 entries per day (365 d/y), 2.105.792.702.478.259 entries per hour, 35.096.545.041.304 entries per minute or 584.942.417.355 per second.

With assumed 600 writes per second (without any reads) you could write entries 974.904.028 years at full write speed. That should be enough.

You could use BIGINT for the primary key. This is a 64-bit number by default.

Edit #2: Apparently what I said before about varying the BIGINT byte length was incorrect. BIGINT is fixed at an 8-byte limit.

Just use 128-bit keys. There is no need for an unlimited number of keys, since you very quickly allow more rows than the number of atoms in the universe. (somewhere around 256 bits).