I had never heard of colcannon before I read this Q. So I looked at a bunch of recipes. Most of them use cabbage, which I think I’d be inclined to do, simply because I love the combo of cabbage with potatoes. Cabbage might be a little more savory, and kale would be slightly more sweet. Some of the recipes call for either cabbage or kale.
I found This One that also calls for leeks, which I think would be a really nice addition. This particular one seems suitable for vegetarians.
Most of the recipes I saw called for bacon, and a few called for ham. Remembering back to my meat eating days, I think the bacon/leek/cabbage combo would be my favorite.
According to This Site colcannon traditionally does not have meat in it. They say it was a dish that was commonly eaten on Halloween and meat was not supposed to be eaten on that day. Be that as it may, most of the recipes I’ve seen today have meat, and maybe even some scallions/green onions as well.
This Site gives a very good description of how to make colcannon (which was a very budget conscious dish) with bacon or ham, it also talks about the substitution of kale, and the addition of leeks. It also suggests that you can use collard greens or mustard greens instead of the other 2 greens mentioned already and it explains how to do that, with slight adjustments. Apparently the spice mace, is another common addition.
I’m thinking that Canadian Bacon might be good too.
Please let us know how you ended up making it : )