It would be useful if the strike counter would show a negative number for how many days one has not been active, instead of just showing 0 (zero).
I think that this would be a really good idea.
I’m not prioritising German at the moment, and am prepared to let it slide a day here or a day there if I don’t have time. However my streak always shows as zero on the drop down menu at the top right in Web. Unless I either remember when my last round of German was (and we all know how days tend to merge into each other), or I actually switch over to that language, I don’t know how many days I’ve let it slip. (Three, as it turned out today, but I thought that it was only two.) A negative number would be more of a “prod” to get me back there.
I think that would be even more useful for people who have a significant number of “minor” languages on their learning list. Negative streak counts would make sure that they didn’t leave any of them too long, and helps if they think they did some work on a language recently… but really haven’t.