Like Oat Flour?
Or give suggestions and pros/cons of other things.
Yes, to all those you've listed!
Go beyond that: add anything with a sugar source, preferably from a range of types, such as fructose, maltose, glucose, and/or sucrose. Try infusing your starter with various aromats. None of the sourdough microflora are native to the sourdough environment, save one (Lb sanfranciscensis), and even that can, through enough successive feedings, be transferred to a non-wheat-based substrate. Most of the organisms have wonderful stress-response mechanisms, which means you can transfer them forward to a wide range of media.
I have done a few gluten-free starters, so all of my experience is with non-wheat grains. Starters do work with grains that are not wheat-based. However, you may need a different ratio of flour to water depending on the type of grain. Some flours absorb more water than others. Oat flour, in particular might be one of those that you would need more water for (by weight) compared to AP flour.
Wheat germ is obviously not something that I've tried but this is the only one on the list that I'm not sure would work. The yeast needs to feed on sugars naturally present in flour. Wheat germ is, well, a germ, not a flour. I don't know if it would have enough carbohydrates for the yeast. If you'd like to have wheat germ in your loaf I'd suggest putting that in with the flour when you make bread, rather than feeding the starter with it. However, if someone else has experience with feeding wheat germ to the starter I defer to their judgment.
Thank you for the info. I didn't exactly explain myself clearly, I was just wondering about supplementing the wheat flour with the other items, because I have them around, and I thought they might add some sort of "micro-special-stuff" to the yeast, and I hate to see things sit around no being used.
By adding (any) whole grain flour, you're actually 'importing' lots of extra bacteria and yeast because they also happen to live on the bran of grains. I guess doing so would speed up the process of getting a really active starter. But like others have said, compensate with your water since whole grains take up more water than regular white flour.
Yes, I think adding some whole-grain flours to your wheat flour might add some flavor to your sourdough mix. Just be aware that whole grains like more water, so you might have to adjust a little. Go forth and experiment!