I think it would be interesting to try and maintain some level of compatibility with xxd. I recently came across this when following a blog post, trying to use hexyl instead of xxd.

• The most obvious thing would be to introduce (yet another) alias for --length/--bytes, i.e. -l in addition to -n and -c.
• Another cool feature of xxd is the -groupsize option. This is already being discussed in Other sizes of data (group size and Endianness) #104.

What other flags/options could we try to support or try to provide aliases for?

What we're definitely not going to support is long single-dash options like -groupsize. Instead, we are always going to keep a style where long options come with two dashes --groupsize.

By the way, one thing that we already do better:

Even though xxd itself doesn't support hexadecimal notation […]

So instead of xxd -s $((0x2118)) ./hello we can simply say hexyl -s 0x2118 ./hello.

Note: we already try to be (partially) compatible with hexdump.