From ones I don't have it leaves Skadi, Angelina, Nightingale, SilverAsh, Schwarz, Magallan, Mostima and Blaze.
I was mostly thinking about Angelina since she's great with her slow and will be handy for stuff like CC with limitations on casters etc. Nightingale or SilverAsh would be great too. Nightingale as ultimate arts defense (which isn't that important until you get enemies like GrudgeBearers/WorldCursers in recent event) and SilverAsh for curing cancer. Of course I can technically get both of them from recruitment, but my luck with top tags wasn't exactly the best. I didn't actually really consider Blaze, but I guess she would also be neat and I did like her in story... Then again I still want game to challenge me.
From that list Silverdaddy would be my first pick; his passives are crazy utility (decloak is a rare, very convenient ability and unconditional global team buffs are very yes) and he ain't known as the Schwing Schwing Man for nuttin'. Blaze is the Final Form of AoE Guards, already a strong archetype, and you can build whole strats around her but just doesn't have the same level of sheer utility and team synergy so I'd rate her as a lower priority. Ange is good too but that no-attack condition on her stronger skills can make her rather awkward to use (S3 turns her into discount Eyja with slow when active tho), and as you say Nightingale's specific niche doesn't come up all that often - her phantoms can be extremely useful though; I got good mileage out of them baiting casters and drones in Anni3 for ex.
Magallan is peak drone gameplay and from vidyas I've seen can be insanely strong if used right, but dronemasters in general strike me as tending to require a bit of rejiggering of your whole tactical outlook and can't really be just "plugged in" to your more conventional lineups on the fly. Wouldn't consider her a priority unless that approach to the game particularly tickles your fancy, personally.
Mostima's interesting. That passive area slow and Caster SP regen buff are no jokes (combo her with Iffy and watch the world burn, for ex, or other area slow sources like Greyy and Manticore to totally tarpit a lane) and her actives are really strong if used right. S2 completely locks down and does heavy Arts burst to everything in her attack range for its (short) duration - I tripped up the buffed Crownslayer in the recent CC with that one to good effect - while S3 is basically Truesilver Slash (Arts remix) with side order of slow and some knockback. The activation costs are pretty prohibitive though, even with the SP gain from her second passive, and she doesn't deploy cheap herself either what with being a high-rarity AoE caster. I'd say she's definitely worth developing and using if you have her, but not someone to go out of your way to get.
Schwarz and Skadi are arguably rather similar in many ways - both are specialised "boss killers" and have the same problem of not really bringing much else to the table, or even doing something that you couldn't just as well use a lower-rarity or more versatile Op for. Franka or Hellagur (maybe even Utage) oughta be able to do Skadi's main job just fine and Hellagood can additionally hold an entire lane unsupported thanks to his self-heal if needed, freeing a medic slot; while Schwarz has no shortage of powerful and usually more versatile ranged monsters for competition. I wouldn't personally suggest picking either in this particular context, and if you somehow had the choice between the two outside it I'd take Schwarz no contest - if nothing else her range makes her more flexible in terms of deployment and application, and Skadi's "potent single-target melee" niche is seriously oversaturated anyway. also Skadi's attack animation looks fucking ridiculous
