do you have any examples? I'll add them to the massive pile...

I'm not who you asked, but I can think of a couple Marx/Engels concepts that have been expanded on by other theorists. What I find appealing about Marxism is that it's approached as a science. If something is wrong, then it is ripe to be corrected, without throwing the baby out with the bath water. The point is for the proletariat to collectively and scientifically develop an understanding of the capitalist world so that it may more effectively advance its class interests.

1. How imperialism works, or more broadly, how capitalist societies relate to one another. Marx was obviously aware of imperialism and colonialism and alluded to its existence, for example in the famous chapter on primitive accumulation. But other theorists definitely made contributions on this front, for example V. I. Lenin, Ruy Mauro Marini, John Smith, Immanuel Wallerstein.
2. How the oppression of women works in detail. Some names are Angela Davis, Silvia Federici, Claudia Jones.
3. Ecology, I honestly haven't read much on this topic though I know it is super interesting, some names are Kohei Saito, John Bellamy Foster