My very first suggestion (as someone who has done a DF youtube series before) is to realise how big a time commitment it is :-) Which is not to scare you away! It's more to say that spending a lot of time planning up front will mean it takes even longer to get videos out. I would just start with something fairly innocuous and go with the flow for the first series. If you look at Kruggsmash's first series, Steelclutches, you can see that it took him a long time to find his stride. In fact, IIRC, the first fortress fell almost instantly to the undead even! He started off being pretty much the only Youtuber that edited his DF play heavily, but I think when he got the story about the goblin that died in the invasion he changed his style a lot. Then we started to see stories about Ital, the god of balance, and finally the Bim Silversnarl adventure series.
In the same way, you should just start and do whatever fortress sounds interesting to you. Or even do an uninteresting fortress and then find something interesting about it. I think this will naturally lead you on to a good path that serves your style well and entertains the viewers the most. If you start with trying to satisfy others, it's easy to chase after stuff that doesn't necessarily speak to you personally.
But if you are asking for what I would personally like to see as a challenge fortress: Satisfy as many dwarfs as you can with their favourite food. I keep thinking about doing a series for that myself, but I have absolutely no time. Probably boring to many, but I would watch :-)