substitute for shiso leaf in umeboshi makizushi
Shiso is in the mint family. Stick to that area and you'll be fine, even though nothing will be exactly right.

Peppermint leaves, lemon basil or Thai basil will all give a nice look and a pleasant flavor, even if it won't be quite the same. A combination of mint and and Thai or Holy basil might be your best bet. A tiny, tiny drop of toasted sesame oil will get you even a little bit closer.

I just read from a totally unknown (to me) source that lemon thyme hits a lot of the same notes, but obviously that would look completely different. Again, perhaps a combination will get you closest.