It would be amusing to retcon him into the backstories of some of the other powerful villains.
Selene had been hiding (licking her wounds) for two decades after being 'destroyed' (she escaped, just by the skin of her teeth, by using a lookalike pawn to die in her place) in Nova Roma after a no-holds-barred dustup with the Blue Marvel in the '60s.
Sinister / Essex *used* to be much more open and active, and even worked *with* people like Herbert 'High Evolutionary' Wyndham when they were hobnobbing around England, being quasi-Victorian gentlemen-of-science, but after they parted ways, his predilections got darker and, decades later, had turned decided nasty (subjecting poor and desperate folk in New York to random mutations, which attracted Blue Marvel's attentions, and led to him deciding to take his operation even further underground than the 'free clinic for the homeless' / 'bargain basement Power Broker' set up he had going on.
When they fought regularly, the Mandarin only used a couple rings at a time. He was still mastering the others and making them responsive to his will (they weren't designed to be used by *humans,* you see, and it's something of a question whether he broke them to his will, or he trained his mind to operate on their wavelength. and now thinks in increasingly alien ways...), and is much more terrifying a foe, these days. (To be fair, so is Blue Marvel, who wasn't nearly as buff, when they used to throw down.)