Not sure if it’s been mentioned before but what if there was another building, accessible only after reaching a certain level, where one could “use up” excess heroes and get random ascension items in return. A 4* hero would yield a 3* ascension item. A 5* would yield a 4*.
Or have a system where multiple heroes’ power is used; a combined power of x gets you a 4* item.
Or even something like x number of 3* heroes would yield 4* item and y number of 4* heroes would yield a 4* item.
Just throwing ideas out there.