If you want to keep a clown in there, you could do a maroon. They're pretty mean, and if bigger than the trigger, won't bother him. Unless he already tends to be a fin nipper. Personally, I think lionfish are fine, even the dwarf. Not sure if the lion barbs can penetrate a trigger's skin, but if so, yes, he'll die. If it doesn't penetrate, can't imagine there would be a problem.
Otherwise, Rabbitfish are fine and some large tangs also. Puffers, groupers, sweetlips (again, if not a nipper...very small sweetlips can be shy), maybe a large angel (not sure about this one, anyone else think that's a possiblity?), filefish, toby's, other puffers (usually get along, especially if the same size). I think that any of these fish of equal or larger size will work. You can probably also tell from the trigger's current behavior how aggressive it will be. I've met some that are picked on constantly by damsels, and never fight back. However, keep in mind that some triggers drastically change personality in their lifetime. Can be peaceful for years, and then turn nasty. Just a thought to throw out there.
Good luck!
Sara