I have a bicycle frame with mounts for both disc brakes and conventional brakes. I'm curious to know what other accessories I can attach to the conventional brake mounts if I decide to use disc brakes instead. Aside from racks, are there any other gadgets or items that can be mounted on these brake mounts?
No matter what you do, consider putting a greased bolt into these to protect the threads. You could even put some kind of cap over the bearing surface to protect them, in case you or a future owner wants to use rim brakes.
The threadded bosses can be used to connect cargo racks. They're quite wide compared to other mounting points, and that helps with lateral stability.
You generally can't use these for mudguards or fenders without making your own or modifying existing products.
Also consider that the bosses are at right-angles to the seatstay. They don't point straight back, so if you try and mount rear lights here, the fitment needs to account for that angle.
I might consider using 3D Printing to add a custom holder for something. A pump peg if you wanted to carry a full sized frame pump, or a holder for a track pump.
One downside is that there is only one boss accessible at a time.. So without two points of contact anything added can rotate. Sure, there's a second boss, but its on the other side of the wheel and you have to go over the tyre with enough clearance.
Thus I'd not try and carry heavier items like water bottles on one mounting point.
You might be tempted to install brakes, on the basis more-is-better, or for added redundancy. This generally doesn't work because you're unlikely to actuate both a rim and disk brake with the same lever, and having two levers on one hand is awkward.
The one bike that benefits from two rear brakes is a tandem, and that's where the second brake is only a drag brake, and is designed to cope with longer hot operation, like when going down a hill. These are often set with a barend lever or something like a downtube gearshift, providing a constant drag for the descent, and then deactivated. They get hot, so only drum brakes or roller brakes are suitable and rim/disk brakes are not.
You might choose to use the rim brake, and install a trailer mounting hitch on the disk caliper mounts.
If your fork crown does not have a hole for accessories (or if it is already used), you could try mounting a headlamp onto one of your front brake mounts (or onto both of them for even more style). Of course you need to make sure there is enough clearance between the lamp and the tire.
Here is an example of the kind of lamps that could potentially fit:
