I have spent the last 20 years brewing beer using pure water, either from RO or distillation, and to me this has become the norm. I like brewing with RO because it takes variability away from water and gives the brewer a blank slate, but as you point out that slate needs to be decorated.
The most important mineral in brewing water is calcium. Calcium interacts with polypeptides and phosphates from malt to affect mash pH, it stabilizes alpha-amylase, is involved in break formation in wort and also influences yeast. You can add calcium from two primary sources; calcium sulfate (gypsum) and calcium chloride. Sulfate and chloride both affect beer flavor and sulfate accentuates bitterness and dryness while chloride is known to give beer a rounder and fuller palate. I like using a blend, and I target a calcium level in most of the beers I formulate of somewhere between 25 and 100 mg/L. If you want an assertive mineral character to your beer you may want to use calcium levels up to 200 mg/L.
While calcium, and to a lesser extent magnesium, causes mash pH to go down, or become more acidic, the carbonate and bicarbonate ions cause mash pH to increase. Most beers brewed in the world tend to be lighter in color and do not benefit from carbonate in water. Darker beers, however, incorporate dark malts into their recipes and these malts frequently result in mash pH that is too low. The ideal mash pH range is 5.2–5.4 and if the pH is too low carbonate species will help move the pH back into this window. When using RO water it is easiest to use sodium bicarbonate as the source of “carbonate” since calcium carbonate is not soluble in water unless the water is acidified. Sodium bicarbonate, on the other hand, is much easier to use. If you want to brew a stout using RO water you probably want to target about 50 mg/L of bicarbonate using sodium bicarbonate.
I named two more flavor active ions in the previous paragraph: magnesium and sodium. Magnesium is one of the key components found in Burton water, although the term “Burtonize” has somehow evolved to mean simply heaping in a pile of gypsum to water. Real Burton water is quite high in magnesium, which is significant because magnesium has a pronounced bitter/metallic flavor and that does indeed affect beer flavor. Magnesium is also a laxative. Most beers in the world do not have much magnesium and I would not intentionally add more than about 50 mg/L of magnesium from magnesium chloride if I were experimenting with this particular salt. Sodium is also flavor active and some people automatically have a negative opinion of sodium because who wants salt in beer . . . or perhaps I should have asked why the German beer Gose has salt added to it in high doses? As it turns out, sodium is not perceived as salty in beer until the content exceeds about 40 mg/L. At lower levels it influences the perception of sweetness and palate fullness. I add about 10 mg/L of sodium with Kosher salt when brewing a variety of beers, especially pale lagers.
Minerals are not the only thing to consider adding to RO water. Lactic acid can be added to adjust mash and wort pH if the pH is too high and the affect that lactic acid has on flavor is completely different than achieving the same change in pH using calcium salts. Yes lactic acid is tart, but that is not what it does to beer flavor when used to make small modifications to pH in the brewhouse. Lactic acid adds complexity and makes a crisper, brighter beer when used in styles that can suffer from what winemakers call flabbiness or lacking in the acidity department.