I think most of these characters wont appear on the show because of copyright and ownership laws. but anyway.
Blue Beetle might be an interesting one, Edward "Ted" Kord could be a man who comes into town who seems like an average guy. He is in his 30s, average build, brown hair, and wears all blue and black. maybe even has yellow shades. He isnt all serious, he cracks jokes now and then, and seems easy going. At the same time a new exhibit comes to the museum featuring a scarab gem. Ted has to protect it from getting into the wrong hands, it belonged to a friend (like the regular back story) and it was lost for years. Ted tracked it down, but so has an evil group who wants to use it.
or maybe he has to take it. He has been tracking down the gem ever since his friend Dan Garret was killed protecting it. Dan used the gem to become stronger, faster, and more powerful to stop evil as the Blue Beetle. Ted promissed his friend before he died that he would become the next Blue Beetle, but the gem was lost, and then stolen. he explains all this to Clark.
At the same time a group calling themselves the Madmen comes looking for the Gem too.
And during this story, Lex is buying the Scarab Gem. thats why its in Smallville or metropolis. He is transfering it to a vault in his home very shortley. Until then it sits in the museum.
It is revealed that while Ted has no super powers, he is an expert in hand to hand combat and also has a few wacky gadgets. He and Clark team up and fight tooth and nail to keep the gem out of the wrong hands.
Perhaps in the end, after a skirmish where several people including Clark almost have the gem, it lands in Ted's hands. In a blinding light Ted is given the powers of the scarab gem and becomes the new Blue Beetle. Using his new powers he sends the Madmen running for their lives. He and Clark part ways after shaking each others hand.
maybe Ted never becomes super powered, he and Clark do fend off the Madmen however. Clark tells Ted, in some subtle way, that he is the Blue Beetle whether he has power or not and that the Gem is still out there in need of protection. And it may need it more now, since Lex has it.
The whole things ends with the Scarab Gem being locked away in one of Lex' many vaults. Across the street, or a few buildings away is Ted, looking through binoculars and standing ready to protect the gem.
then again
it could be Dan Garret, while Id prefer it was Ted, it could be interesting to see a man come to town with incredible powers. He wears all blue and some red. He is super strong, fast, bullet proof, and magical. Its all because of the Scarab Gem, he has been on the run most of his life protecting it from getting to the wrong hands. This time a group called the madmen are after him.
its sort of like an Indeana Jones story, high adventure.
He runs into Clark, but he claims its not by accident. The Gem must have guided him there. he needs Clark's help, he could mention the other cities he has been in like Gothem and Coast City.
At the same time Lex meets with the Madmen, who he doesnt know is a terrorist group. He makes a deal with them to get the gem, for his collection.
Clark, hesitant at first, helps the Dan "Blue Beetle" Garret. But something goes wrong and the gem is stolen during the fighting. Dan becomes human again, but with Clark's help he keeps going and gets back the gem.
At the end he has to leave, he has endagnered Smallville enough. He must move on, its his curse. Clark mentions that maybe theyll meet again some day, Dan tells him you will see the Blue Beetle again in one form or another.