Alright then, I'm going to take a dive into the options I'm liking the look of (mostly looking into all the combos of all species and specialties would be far too long).
I am not particularly interested in the Hicys or the Tortolan. For the fomer, the plant and fungus magic just doesn't seem to gel well with any of the classes except maybe alchemist, and I'm not really a fan of RNG in our core skills. I'd be perfectly fine with the reduced speed and forced natural armor of the Tortolan, but combine that with our armor having built in bypass points and making so much stuff just cost more turns me off it.
Of the rest with the two high level classes, that gives:
Dark Unholy Souls: Bhomon/Apostate. I think
@Metarite summed it up pretty well. Very low HP, and most of what we got is reserved for casting spells anyway. Better work on those dodge rolls.
Fallen Fox-cat: Felixar/Apostate. Not much to say about this combo other than it exists, but the class negates the species Max MP negative, and with the boost to climbing and stealth it's probably the best option we have for hiding from conflicts caused by our specialty.
Night Halflings: Bhomon/Astral. I can't really say much off the top of my head about this combo either.
Night Predator: Felixar/Astral.
@Metarite summed up this one pretty well too IMO. A sneaky burst caster striking from ambush and hoping to kill you before they run out of MP. I think there's a significant amount of leeway that may come into play with the backgrounds though. I could see the Eclipsed Orphan or Station Rat backgrounds leaning more towards the "sneaky" side of sneaky caster, while the noble or temple orphan backgrounds may run more on the caster side.
Finally I think Human/Eclipse/Astral deserves an specific mention besides Human just slipping into any build really, in that it would give us a random species ability tree, a random background passive, and a class passive that changes depending on where we are and what time it is. If we want to keep the dice rolls going for the rest of Char-gen and have some variation in the actual game without too much randomness this is the character for that.
[X] Plan: Unholy Souls
-[X] [SPECIES] Bhomon
-[X] [BACKGROUND] Noble Scion
-[X] [SPECIALTY] Angelic Apostate
[X] Plan: Fallen Fox-cat
-[X] [SPECIES] Felixar
-[X] [BACKGROUND] Eclipsed Orphan
-[X] [SPECIALTY] Angelic Apostate
[X] Plan: Night Predator
-[X] [SPECIES] Felixar
-[X] [BACKGROUND] Eclipsed Orphan
-[X] [SPECIALTY] Astral Mage
[X] Plan: Noble Night Predator
-[X] [SPECIES] Felixar
-[X] [BACKGROUND] Noble Scion
-[X] [SPECIALTY] Astral Mage
[X] Plan: Two and a Half Mystery Boxes
-[X] [SPECIES] Human
-[X] [BACKGROUND] Eclipsed Orphan
-[X] [SPECIALTY] Astral Mage