Triggers often can often be mixed if they are very different and are added together. A Odonis Niger and a humma could co - habitat (I have that exact combination in a tank at the shop right now) but with triggers, you never know what they might do. I have had triggers together for years with no issues and then one decides to be a jerk. I have a humma in with a clown right now at the shop too (not a combination likely to work) but somehow am getting away with it. Blue jaws (throats) are always pretty peaceful, Pinktails are normally peaceful although we have a pretty aggressive one at the shop right now. Crosshatch are very laid back but are expensive.
Angels are a great addition but they vary in attitude. One of my favorites that is typically peaceful is the Navarchus, some times called the blue girdled or majestic. Same with the blue face. Emperor's and most others of the pomacanthus are a little attitudy (is that a word?) but well worth it. Queen angels are awesome and typically don't cause to much problems. Passer angels are flat out mean, need to be kept with other meanies. I have a male Scribbled angel right now that is awesome and seems to be mild mannered (as they typically are)
Wrasses are cool too but again vary in temperament. The aforementioned Harlequin Tusk (another one of my favorites) is very peaceful, even kind of wimpy where as members of the thalasoma genus are typically mean (lunare for example) If you like the torpedo shaped body with lots of action but don't want the aggressive temperament, green bird wrasse are nice, so are male paddlefin wrasse to mention just a couple.
Lion fish are a great addition but it's sometimes hard to categorize what type of tank they belong in. They are not mean or dangerous like their undeserved reputation. They typically should be kept with larger, mild manored "flat shaped" fish like tangs or or angels that would not be easy to swallow. Many wrasses are slender or "torpedo" shaped and a lion will sometimes gulp them down because of their easy to swallow shape. Triggers are "iffy" because they often (not always) will nip at the long, flowing fins of lion fish. Be sure to get a lion that is already eating frozen food too.
Puffers are a great addition although as mentioned before, the porcupine & spiny box get nipped at by many fish. The members of the arothron genus are a great group of fish. Some get quiet large but others, like the dog face (nigropunctatus) are very manageable. If you want one less common, the melegris (especially the gold phase male) are really cool. The Mappa that Gary (viggen) just got is awesome but it's gonna get big!!!! Narrow line puffers or stallatus puffers are cool too and don't get huge. The Stars and stripes (hispidus) might have the best personality of all the puffers but can reach 10-12" in a year or so and keep on growing to 18 to perhaps as large as 24".
Groupers can be a cool addition if your keeping fish on the more aggressive side. My favorites are the Louti & the Miniatus. They are very colorful, hardy and disease/parasite resistant. (dont keep a harlequin tusk with either of these) There are several others too that are equally desirable.
Don't forget eels. There are some very peaceful eels that are very attractive like a golden from Brazil or a Zebra Moray. Both are easy to keep and mild manored. Just give them a good hiding place and a secure lid on the tank. There are plenty of other eels available, some can be pretty nasty but several make good aquarium residents.
Well, that's what came to my mind, hope it gives you some ideas.
See ya.....