Off the top of my head with your restrictions in mind. Other people will do you better, tbh.
Tank: Paladin/Chanter. You can go goldpact and a troubadour just to continue applying various buffs, but it's whatever because this multiclass is pretty strong in a party. It's got armor, auras, sustain, damage, and possibly bonus engagement. Just remember that auras count as active abilities, so certain bonuses will not stack with things like modals, spells, or potions. You can still stack equipment, though, and if you stack every available regeneration accessory...
DPS off-tank: Fighter/Monk. You can do a devoted helwalker for some funny damage. Access to tier 3 might inspiration and stacking +2 pen on PotD is really good. Choose mob stance for dumb results. Tactician is also really powerful in a party. Consider the benefits. If you don't go devoted, consider using the ability, clear out, w/ weapon modals to debuff secondary defenses by 25. You'll want to look at club, flail, and morningstar. This significantly bolsters your allied Magic DPS.
Ranged DPS: Cipher/Rogue. You can kind of wing it here in subclass. Rogue abilities are powerful and you only ever need hobbling strike for damage in most scenarios. Fire off Cipher abilities for phantom foes or something. Don't forget to take Cipher's talent for +1 pen on weapon attacks. This multiclass will work well with whatever firearm you want them to have, pretty much, but works best with dual pistols. Just don't forget to take Maia's sharpshooter's garb for the extra reload reduction on arquebuss since that's what you desire.
Magic DPS: Wizard/Barbarian. Just do bloodmage. The barbarian should be either vanilla, berserker, or fury shaper. Berserker might do too much raw damage to you but with proper party support you'll be groovy thanks to bonus pen and armor. Frenzy for bonus action speed. Hilarious. Also, this multiclass is flexible enough to use a conjured weapon if you're so inclined.
Healer/Buffer: Unfortunately with the conditions you've set, the last option is Cleric/Druid. This is not recommended, but I guess if you're just looking for healing and buffs, this will have you covered. Combined with the Tank's auras, you'll pretty much have healing as good as it gets in Deadfire aside from what a certain user on here would do (team of 5 paladin chanters). In fact, you'll probably end encounters before you'll need this multiclass... and when you do need it, it'll be for barring death's door.
Edited by UltimaLuminaire, 28 February 2019 - 02:07 PM.