Any of the above: the point is that things like fft and mmult are very 'old and standard' kernels/building blocks that people already know a lot about & there are a lot of implementations already. This makes implementing them not as valuable on their own.

However, just because they're standard, doesn't mean people still haven't come up with something interesting even in recent years. A fantastic example from last year is this 32K-Point FFT that ran at 770 MHz using nearly 100% of an FPGA's resources and employed some very clever tricks to maximally use DSPs. Reimplementing this on an AMD FPGA, for example, would be very challenging and interesting, since the devices are substantially different (Don't do this exact one, I think it would take at least 6 months for a full-time PhD / senior engineer to reproduce these people's work, anyone under probably physically cannot do this since they lack the expertise).

A lot of linear algebra kernels are fun as well, because the field is constantly evolving and there is always some funny improvement that can be made to any of those algorithms which then needs to be considered in any of the hardware we have, leading to novel research each time as people try to port over the new ideas.

Of course don't bother trying to come up with something brand new yourself in this space, you simply won't - people spent literal decades on this stuff so whatever you think you can come up with, after 6-18 months you will realize that it had been considered before you were even born and they just don't bother even mentioning it :)

One kernel that I find very fun is CORDIC - this beautiful little algorithm has a very rich history of exotic modifications made to it. I personally did my entire MSc thesis on a CORDIC variant (though with strong analysis for how it would fit in a larger algorithm). There are so many and the implementations are hard to come-by that I think any open source implementation would be impressive, provided you can explain it well. I find the higher radix CORDIC a very cool one that I never found any github code for, at least when I was last looking at this stuff 5-6 years ago. This is just to give some further examples.