My question is how does a brewer develop great malt aroma in beer? Is it the type of malts used? Is it the brewing process? Is it a water profile? I'm sure the answer is all of the above, but I'm wondering if there is a major factor in overall malt aroma? My lagers tend to have a much better malty flavor than a malty aroma. I use fresh, quality ingredients and have even built my water up from RO, so I'm at a loss.
I think you've got the basics. Fresh malt is important. Experiment with different maltsters and grain varieties. I think you tend to get a higher quality malt aroma from some of the English, Belgian, and German maltsters than North American ones, but some are better than others. You might find you like the Pils malt from one maltster but the Munich malt from another.
If you want to increase the malt aroma and flavor in a beer, you can try swapping out some of the base grain for malt with more aromatics. Use Munich and Aromatic malt. Possibly try a no sparge technique. Don't feel constrained to a "classic" grist from a specific country. I have no problem throwing German Munich malt into an American pale ale or German Vienna malt into an English barleywine. You could also blend different grains from the same class to increase the malt complexity (if not the intensity) -- that could be perceived as having a higher malt aroma. For instance, using Golden Promise along with Maris Otter, or adding some continental Pils malt in with your 2-row. I like using German Vienna malt in a blend with English Maris Otter in some styles. It takes some trial and error to find the malts that appeal to you, but when you have them identified, then try some different grists to see how you like the combination.
You could also try decoction, even in styles that don't traditionally use it. Any time you encourage the Maillard reaction, you're going to get increased malt complexity and richness. Caramelization is a different process but can also give you those characteristics. The Scotch ale technique of boiling down first runnings to increase maltiness is something you may want to explore. Just keep in mind that whatever increases aroma is likely to have a greater impact on flavor. It's certainly possible to create a beer that's too malty in the balance for a specific style. I've tasted some doppelbocks that have so many Maillard products in them that they kind of taste like beef broth.
I think the water profile has less of an impact than the other factors you listed. Also keep in mind that oxidation can both help and hurt the malt aroma. I say it can help because low levels of oxidation can give honey, caramel and fruity notes before you start getting stale and papery qualities. However, I think you are trading intensity for quality if you go that route. In general, oxidation can mute and muddy aromatics, so it's best avoided.
The perception of aroma is somewhat temperature dependent as well. So if you want your beer to have a more malty aroma, you could also serve the beer at a warmer temperature. Different glassware can also emphasize the aroma, so you could also look into serving your beer in a different glass.
I wish there was something more formulaic for you to follow, but it's really not that easy. Aroma depends on a large number of factors, so you may need to fiddle around with several aspects of ingredient selection, ingredient handling, brewing, packaging, and serving to get where you want to be.