My knowledge is limited and I've got maybe 20 games of malifaux under my belt in total so please take my input with a grain of salt. I've really only any experience with Tara (I have the Viks though not used them yet) but Convict Gunslingers are great for me. I bought them to use alongside the Viks to bring range to their crew but maybe Freikorps can help you with that so I wouldn't cite them as a 'must buy' but something to consider as a future purchase. They put out a lot of damage with rapid fire and positive flips so I'd speculate they're better than what comes with the VS box but I'd do due diligence and make a judgement call.
Then there's Johan who I own but I've not used him yet. Looking back on the last few games I've lost, I've concluded that he would have been a much needed asset for what he brings to a crew. The number one reason being 'Rebel Yell' which removes conditions on a target model. This can be used on yours or your opponents models so he is very useful for that on its own. Additionally, he also gets bonuses when facing Tyrants (Tara, Hamelin and Rasputina, maybe others) and constructs which a lot of factions use. I'd highlight him as a must have purchase for the reasons above but he's also no slouch in combat anyway so I think he'll find a place in many crews.
Another notorious Outcast is Bishop. He's killy and adaptable to all manner of opponents but that's really all I know about him. I'm looking at picking him up for my Tara crew but I think you've already got plenty of what he provides with a Viks crew and a VS crew would be better served by Lazarus.
Sue is a souped (sueped?) up Convict Gunslinger. I've not obtained him yet but he's on my purchase list as he brings decent ranged firepower with tactical actions that allow you to draw cards and give 'finish the job' to other models helping with getting scheme markers onto the table. I'm a big Johnny Cash fan so I might be biased but I definitely think he'll be worth a look for any Outcast master.
Now on to the elephant in the room - Killjoy. I like him in theory but mostly because his reputation proceeds him and I feel your opponent will over commit to take him down. I've used him with Tara twice but on both occasions he was taken down relatively quickly by my opponent who ran a Sonnia crew. I'm in two minds because his death always bought me breathing room but I grudgingly feel he wasn't worth the 12 soulstones cost. I'd list him as a 'your mileage may vary' due to your opponents knowledge, crew and if you can reliably get him out early enough to impact the game. I need to play test him more myself because I think Tara's unbury mechanic works well with him, but with the Viks I feel he would be overkill and I don't see any real synergy with Von Schill.
Honourable mention should go out to a few others;
- Aionus; if you can still pick him up, he hands out slow to opponents and fast to cheap minions. He also has limited bury tricks but I like him because he can move scheme markers 6".
- Freikorps Strongarm; gets a lot of opponents in a spin and is thematic with Von Schill but I've never faced one and don't really know anything about Mr Moustache and his crew. Worth a look though.
- Ashes and Dust; great scheme runner that when is killed, splits into two parts that can reform to remake A&D. Expensive and not sure if suited to your current crew boxes.
- Malifaux Child; cheap totem that can copy a masters spell with a plus to cast and a negative to damage (or vice versa - I forget). Ive only faced him with casting opponents. Not sure he'd have much use with your current crews.
- Desperate Mercenaries; I wouldn't bother, they're not bad per se, it's just there's much better options if you need bodies in your crew.
Finally, and apologies for what has turned into an essay, a word of caution with Outcasts from one newbie to another. The crews you have now are good, but pretty much any other outcast master you pick up, will end up needing resser minions (or 10T in Misaki's case) and that becomes a slippery slope. Even Hamelin will benefit from crooligans and canine remains. Just something to bear in mind when considering future crews. Hope this overly long post guides you to making informed future purchases and welcome to the Outcasts, you're in good company.