Is there a discipline studying the prioritizing of repeated tasks?
By far the simplest solution is probabilistic. Let's suppose you have $k$ classes, and their relative importance is $c_1,\ldots,c_k$. Let's also suppose that there are $N_i$ objects in class $i$. You want the frequency of appearances of each symbol in class $i$ to be proportional to $c_i$.

Let $P = \sum_i c_i N_i$ and $p_i = c_i N_i/P$. At each step, choose class $i$ with probability $p_i$, and output a random element from the class. The probability that a specific element from class $i$ appears is $p_i/N_i = c_i/P \propto c_i$.

To increase fairness among class $i$, instead of outputting a random element, you can keep a counter for each class (counting modulo $N_i$), incremented any time the class is chosen, and output the corresponding element from the class.

A similar approach should work for fairness across classes, but here it's not as straightforward so I'll leave it to you.