For example like an IDE one should be able to easily see definitions defined somewhere else. Right now it is doable only if an author spends a lot of manual effort.

You mean like how Wikipedia renders the beginning of a page if you hover over a link in a wiki page?

Sure floating tooltips or "show and hide" buttons are possible ways to let one preview cross reference within current context instead of leaving the page. This would reduce friction and memory load. This book https://mphitchman.com/geometry/ extensively uses this and other (inline exercises, e.g.) through the pretext markup scheme and tools. As a first step it would be nice to have some of these features working in the more popular md schemes. But I also happen to think that it is possible to go beyond. Math could use a better semantic markup that enables better presentation of underlying structures in addition to pretty tex formulaes.

I would like to see a math equation traceback. Like for a stochastic model of animal population distribution you might have something like

p_ih = p_0exp[(ad_ih)^c + (b*s_star)^e]

and it would be really nice if you could just see the traceback on everything, like you click on s_star and it takes you to the definition

s_star = (s_max - s_h)/(s_max - s_min)

and you can keep going until you are done. I guess I'm just tlaking about a wiki at this point but the focus would be on integrating the definitions into the equation itself so that when you read it in a textbook or whatever you can see where the ultimate comes from.