I'm only here to talk about
What made him so memorable? What gave you the impression that he was a real and believable villain?
One thing: Ron Perlman's voice.
He doesn't just voice Slade, he IS Slade. Any other voice would not only sound wrong to my ears, it would sound false. It matters that much.
Do yourself a favor developers. Cast this character correctly. Get Ron Perlman, or don't bother getting anyone. There is no reason NOT to pay this man whatever his fee is for voicing Slade. There are millions of reasons (can be taken as the money you will make, or the customers you will piss off) why you should. Don't fuck this up. You will regret it if you do, and in the worst place possible for a developer; on your bottom line. Casting anyone else for the voice of Slade will cost you in the end. Money, and fans.