Honestly, I don't really have any ideas, so the floor is open to you. Have at it.
100 % sure ? I'm fully open to any ideas you might have.
Well, to avoid keeping you guys waiting, here are my suggestions:
Colonies of Slavic nations other than Russia in Asia - Fairly straightforward. Make flags for OTL or ATL long-term colonies in Asia that were created by any of the Slavic nations of Europe, excepting Russia (to make the challenge a bit more tricky). Which part of Asia the colony is founded in doesn't really matter. You can go slightly ASB if you can't think of any other solution, but try to keep your entries at least somewhat realistic.
Flags of alternate Bantu nations/countries - Take an OTL or (at least slightly) ATL version of one of the many Bantu nations of subsaharan Africa, and create a flag for their ATL sovereign/independent country (pre-colonial or post-colonial). Note that the country can be somewhat based on an OTL one (Botswana, Kenya, Malawi, etc.), but has to have a different name and different borders. Also, OTL South Africa's "bantustans" don't count as an example, it has to be a proper country of its own. An ASB/fictional basis, e.g. Marvel Comics' Wakanda and the like, is not allowed either. Kindly avoid the pan-African tricolours if you can, they're boring and would be too parallelistic/anachronistic in most pre-20th century timelines.
So, guys, tell me which of these you'd prefer.