A notification per day, at noon (I guess others would choose another time), about what remains on schedule for today and how much remains to keep the streaks alive IF anything remains and the minimum for streaks haven't been reached.
I usually do it in the morning, so if by noon I'm not done, a reminder would be welcome. I also use Duolinguo, for Greek, and I find a bit absurd to have a notification when I wake up, I mean, let me have breakfast, and wash, maybe, before bugging me?
(and I won't get to it before I finish my Renshuu schedules anyway).
About the streaks, it's completely silly, but I've been doing my kanji schedule for 3 months now. But once a month the schedule is under 25 kanjis, and I don't notice, and it breaks the streak. It's silly, but it's a small disappointment when it happens.
And another notification per day, at the same time, IF there's a new message / forum answer.