Yeah they could have used Bizarro but really seems like a waste. There is a decent amount you could do with Bizarro from a character interaction aspect to his entire world. You can go comical with it or really dramatic. Would have been a waste. Bizarro is an actual character that can actually have an arc as shown in the comics, cartoons, and hell even Smallville. Bizarro is someone you can write an entire movie around. Doomsday just is a monster and a weapon. You could expand him a bit more sure but only a little bit. In the end he's the still going to be that weapon and monster. He's a tool to be used by another villain or some force of nature that happens to get free.
Also it's not like he's gone for good. If Superman can come back to life he certainly can. Hell if they're following the comics, no reason to think they're not, he's not actually dead. Comic Doomsday didn't die as a result of his fight with Superman. He was just knocked out long enough for him to be captured and then tossed into space by Cyborg Superman I believe. Smallville just has him trapped in the end. I actually liked how they did that there. Clark and crew trap him deep underground in the end. Which sort of sets up a potential Death of Superman angle later down the line where in comics Doomsday is trapped deep underground and breaks his way out and resurfaces. Meaning that scenario could potentially happen in the Smallville universe at some point. Wonder if the season 11 comic ever touched upon that.
