Quirk wise he could use the tank quirks as he has a self heal melee attack to be a tanker as well as a few trinkets to buff his self heals. All damage type quirks depending on your focus. (Ranged and Melee) could even go both with Unnerring and Slugger with Deadly quirk so you have 2% chance to crit on whatever your using.

My Slayer is always on 2nd position using melee skills most of the time. He is complete self-sufficient with Biting Blade, Rip and Tear, Run and Gun, and Reeling Reload. Yep, I don't use BFG lol. Even Reeling Reload is usuallu used just for returning to 1-2 position whem ambushed or so. Rarely I use Rund and Gun to finish off quickly some squishy or almost dead enemies. I use Slayer in pretty mobile parties including Shieldreaker, Hellion, and so on. As for quirks... Well, Darkest Dungeon doesn't offers much to be really effective. On the first place for almost every character (especially for damage dealers) are quirks Eldritch\Mankind Hater\Slayer (I give priority to Hater quirks 'cause of stress resistance). Due to pretty good stress management and pretty high crit chance I try also to lock on a doomguy quirks like Resilent (+ stress healed), Steady (- stress), Gifted (+ healing received) or Photomania (- stress when torch is 75+). Oh well, you can't lock 'em all without mod and I don't use such kind of mods :) So I lock what I can find huh

4fold is quite possibly the most busted trinket in the entire game, modded or not. HP damage is easier to heal than stress dmg, and 95% of all hp damage can be mitigated by sticking an MAA/HM on him. Slap on a Periapt (200% dmg - 35% dmg is still 165% dmg) and he becomes an absolute monster that oneshots corrupted giants. Suggesting nerfs (maybe +stress dmg, or 4fold to inflict bleed/stress/blight/whatever when the slayer attacks).

Party idea: position 1 Slayer, position 2 Highwayman, position 3 Musketeer, postion 4 Shrapnel. There, an all guns team is made with no repeated classes. Don't judge me for liking Muscarine's classes, they have legit interesting gimmicks.

He's made for the endless dungeon of CoM, capable of reaching up to 1000% extra damage and 78% crit on Bound for Glory when reached the third boss without camping. Quirk wise tanky and accuracy quirk is essential, the best trinket for him imo is the Distended Crowseye from Shrieker for it 15 accuracy, all the accuracy is just to guranteed the 150+ damage bound for glory to hit

Biting blade, spike sabot, rip and tear, bound for glory (I prefer not to use move back skills)I don't have CoM so I wouldn't know a good position for that. I like to run him in poistion 2, highwayman position 1 (with Point Blank Shot), vestal/occultist and then your choice (I choose Jester for easy destress). His spd is really slow, so I put on a spd trinket and the ancestors gun for high accuracy! Stack up atk and crit. Don't use camp until you use your Bound for Glory, making an easy Shambler/boss kill!

the only thing i would change from this character, is to allow him to be able to use run and gun from position 1, 2 and 3, instead of just 2 and 3

Overall I like the character. The art is amazing, and abilities have some decent synergy with each other, and offer two clear party roles for a character, which is something I like with a mod class. But I do think Reeling Reload should be useable on any enemy rank. Without a movement capable party in the middle, by the time you want to move the Slayer back up (assuming he starts at position 2) in a fight, the entire back line is usually completely dead and you can't reel him in to get up close and personal because theres only 1 dude left for a target and they're standing in the front.

i really dont see a reason why his mark should not be available position 1.
He himself is not a mark character. He also gravitates heavily towards position 2 where all but one of his skills are available. Position 2 is a great place for a lot of modded and base game mark heroes to be. Id like to run a melee slayer that can help by applying mark from up front