I really don't care how hard/long/whatever the challenges are. But i don't want to be forced to do them constantly or i'll 'miss' the rewards.
Honestly if the series are made to not be single-time and their standing to accumulate through the runs, that would be one of the cleanest solutions. Have multitude of series running and cycling through eachother. Similarly to operation Plaguestar or the ghouls - they appear once in a while, you do some stuff for them, but dont lose your progress the next time.
On the side, I do like the daily/weekly system - finally something else than sorties thats worth doing on daily basis. Though a monthly challenges would also be welcome.