Penance13

Shapeshifting is easily the best bet for a disguise/ secret ID, Depending on how complete it is. Beast boy is a shapeshifter, but he's always green...

With anything else, someone could possibly put 2+2 together if they pay attention, or just cheat and do mind reading...lol.

Like the one guy in Dark Knight who figures out that Bruce is the Batman by looking at the cash flow into certain areas, and the real life uses Batman has for them.

The first x-men movie toad was what toad should be like in the comics-not super powerful, but uses his minimal powers pretty 
effectively.

Juggernaut maybe-not sure if red can drain magic.

Not sure-It might not be a matter of power, but of how you use it.  A case of raw power V.S. Experience & skill

Joker would be good-I've also heard someone mention Jason todd as a yello lantern but he's more of a red guy...red hair, red hood...red ring..lol
 
Psycho pirate would be another good candidate for the yellow lantern corps

Arkillo & Sinestro of course

Todd's one of my favorite DC characters-once he put on the hood and even more when he dressed up as batman and started shooting at people. 
He would make the perfect villian for dick-or he could finally move on to bigger & better things.
 
And I would imagine he'd be more of a re lantern than a yellow one...but I see your point.

Shazzam!!!   Hahaha

And that's why it's a bad idea to go toe-to-toe with black adam

I have a lot of pent up anger, but it's hard to make me show it... when I do it's always a bad thing.