Sometimes, one has many more ideas than time. Or a great idea that does not fit your style, your category of art (photography, 3D art, traditional art and so on). Or an idea that many people could use at once and it would still be interesting. Why not share it with others?
I have taken part in challenges and I have even seen people using their entries to create series. A well known example is the Inktober. But I have seen many more.
January - Creatury (draw mythical creatures)
February - Faebuary (draw fairies)
March - March of Robots (draw robots)
April - April Shower (draw something rain related)
May - MerMay (draw mermaids)
June - Junicorn (draw unicorns), Junebugs (draw bugs)
July - Julycanthropy (draw werewolves)
August - Smaugust (draw dragons), Fawngust (draw fawns and forest animals)
September - Sketchtember (draw one sketch every day)
October - Inktober (draw with ink), Drawlloween (draw something Halloween related)
November - Huevember (draw a picture with only one color)
Dezember - Drawcember (no topic, just draw something every day)
But aside from premade challenges, you can create your own ones or search online. I have created two that were originally meant for character design, but can be used for anything else as well.
30 food-related pictures
30 Liquid-related pictures
If you have ideas or are looking for some, feel free to join in.