Yes...it is something I have thought about for many years. More significantly to me, it is something which I have actively pursued hands on in my garden by running numerous "experiments", all trying to find the best natural sources for Nitrogen and for overall soil health and well being.
Before listing out my preferred change agents a couple of comments.
1) it isn't a short-term effort or shouldn't be viewed as such. Rather, it should be a lifetime commitment or failing that as long as you can,
2) it isn't just about NPK. Micronutrients are also very important and soil tests are the best to determine your situation.
One more comment before giving you my list of preferred change agents: soil renovation/maintenance accomplished through the use of so-called cover crops is the absolute best, most effective method. It would take me many pages of text to elaborate on the experiments I have conducted and am still conducting on rebuilding soil NPK and micronutrients through the use of cover crops. You can find some of those results on this Forum, if interested
Now, my list, absent what I consider by far the most superior agent, cover crops.
1) alfalfa meal
2) bone and blood meal
3) fish emulsion
4) manure
5) coffee grounds
6) grass clippings/leaves