What is a good replacement for a cable end cap?

You could maybe cut it where it is not frayed and put a cap if the slack is long enough.

Or you could put a piece of duct tape just to avoid the fraying from reaching a place interfere with the derailleur.

Or you could try to gather the frayed-out strings and put a cap anyway

Or if you really don't like it, you can change the cable completely though it is overkill.

Those end-caps are quite reusable. If you take a pair of pliers and squeeze it gently so that the flattened portion opens up you can then slide it back onto the cable. Using the pliers you can then re-crimp it on the cable. Although it looks ugly and everyone will know that you are too cheap to buy a 10c end-cap.

A little bit of superglue placed at the end of the cut seems to help keep the individual wires from separating.

If you have an unneeded old wheel laying around, use a spoke nipple. Remove the nipple from the spoke slide it over the cable and crimp with pliers.

I have successfully used A tiny bit of epoxy glue.

Another option is a bit of tin soldering wire: give it a couple of tight loops around the end of the cable then squash it with pliers. Tin is soft enough to be defformed around the cable without deforming the cable.

Yet another option is to use an actual soldering iron to cover the very last centimeter of cable with tin. It is quite difficult to solder the end of the already installed cable though.

If the cable is not too badly frayed it can be de-frayed with pliers, grabbing all the strands and twisting it in the right direction. Do it before applying the cap end substitute.

I've used a cable crimp. It looked terrible but it prevent the cable from unravelling before I got to the bike shop to discover that a bike shop will usually give you those end caps if you buy something else from the store. The shops have them in the thousands and they probably paid about $3 for them.

If you have slack in the cable, cut the frayed end and cap as usual.

If you don't have enough slack, use JB Weld to prevent further unraveling. Work it into the fibers, and wrap with duct tape to hold tight while it cures.

In a pinch, use wirenuts. They're available at hardware stores. The smaller ones work better as a cable end-cap.

I've used these cable connectors before...you just break one off and screw it over the cable end. They work great and look pretty "impressive".

Cut a bit of cable housing, peel the plastic, and squeeze as you would a conventional cap (using more force, though). Doesn't look bad but is no substitute for actual end caps.

You could use some heat shrink tubing (example) to keep the frayed cable together.

If you don't have a heat gun, most heat shrink tubing can be shrunk with a hot hair dryer.

I had a similar question but for installing an internally routed dropper post: both the remote lever end and the post end require a cable stop end (not the crimped on kind to prevent fraying, the tension-holding kind). It seems this sort of thing is sold both specifically for bikes and for automotive/general work.