Anything other than water, flour, and yeast is an "improver" -- yes, even salt, which is 1) a flavor enhancer, 2) yeast retarder, and 3) gluten strengthener. Fats and sugars tenderize and brown, egg whites can strengthen, while yolks can tenderize and slow staling. Most flours are milled with acsorbic acid and/or non-diastatic malt, so not much point in searching for those. Lecithin goes bad easily, and can often be spoiled before even sold, so be watchful (the effect of egg yolk is primarily from the lecithin, and so I forgo refined lecithin altogether). Most of the others are kind of pointless, as their effects are small compared to the "artificial" additives and preservatives available to commercial bakeries.

One thing I would add - it is worth getting hold of strong baking flour for the dough, you will get better results then with general-use flour.