I'll think out some ideas a bit more and why I think they could be considered:
1) Centaur
https://forgottenrealms.fandom.com/wiki ... ?so=search
+ Would make a nice fit for cavalier
+ Flexible for all sorts of RP, not stereotyped, good or evil
+ Fits well with the wilderness theme of much of the surface
2) Were-creature that isn't just a werewolf and can shift at will and retain PC control, but also shifts as per lycanthropy and loses PC control. Would be non-curable and spread the curse as per lycanthropy, remaining curable for any other inflicted PCs, but with a reskin to whatever the base were-creature is.
+ Gives a 5% option without making Arelith look like a zoo
3) A possessed existing race. It would function exactly like number 2 above, except you could be possesed by a ghost, or demon, or devil, or whatever is appropriate and wouldn't be contagious.
+ Gives a 5% option without making Arelith look like a zoo
4) Azer
https://forgottenrealms.fandom.com/wiki/Azer
+ Flexible for all sorts of RP, good or evil
+ Gives Brogendenstein some 5% love
5) Arctic dwarf
https://forgottenrealms.fandom.com/wiki/Arctic_dwarf
+ Gives a non-evil or non-monstrous 5% option
+ Presumably easy to introduce, probably just tweaks to the Wild Dwarf template
+ Fits with the large cold wilderness areas of the surface and Skal in particular
+ Gives Brogendenstein some 5% love
[Edit: actually thinking about it, would the DM team consider Wild Dwarf and Arctic Dwarf similar enough that it's permissible to play a Wild Dwarf as an Arctic Dwarf if you take the gift of hardiness?]
6) Bralani
https://forgottenrealms.fandom.com/wiki/Bralani
+ Gives a non-evil or non-monstrous 5% option
+ Gives a 5% option without making Arelith look like a zoo
+ Gives Myon some 5% love
7) Celadrin
https://forgottenrealms.fandom.com/wiki/Celadrin
+ Gives a non-evil or non-monstrous 5% option
+ Gives a 5% option without making Arelith look like a zoo
+ Gives Myon some 5% love
8) Abbalaya
https://forgottenrealms.fandom.com/wiki/Abbalaya
+ Gives a non-evil or non-monstrous 5% option
+ Gives a 5% option without making Arelith look like a zoo
+ Gives humans an interesting option
+ Subtle, maybe give them Divination foci, DC bonuses or innate spell casting
9) Sahuagin
https://forgottenrealms.fandom.com/wiki/Sahuagin
+ A good fit for Sencliff pirate RP
+ Gives Sencliff some 5% love
+ Introduces a fun source of conflict and balance for the many Sea Elves
10) Selkie
https://forgottenrealms.fandom.com/wiki/Selkie
+ Gives a non-evil or non-monstrous 5% option
+ Gives a 5% option without making Arelith look like a zoo
+ A good fit for Sencliff pirate RP
+ Gives Sencliff some 5% love
+ Gives 5% options to those who want to play non-evil sailors
11) Sirine
https://forgottenrealms.fandom.com/wiki/Sirine
+ Gives a non-evil or non-monstrous 5% option
+ Gives a 5% option without making Arelith look like a zoo
+ A good fit for Sencliff pirate RP
+ Gives Sencliff some 5% love
+ Gives 5% options to those who want to play non-evil sailors
12) Tabaxi
https://forgottenrealms.fandom.com/wiki/Tabaxi
+ Everyone wants to play a cat
+ Flexible for all sorts of RP, not stereotyped, good or evil
- We all know that for every seriously played Tabaxi there will be another kawaii catgirl or furry
13) Dryad
https://forgottenrealms.fandom.com/wiki/Dryad
+ Gives a non-evil or non-monstrous 5% option
+ Fits well with the wilderness theme of much of the surface
14) Half-dragon
https://forgottenrealms.fandom.com/wiki/Half-dragon
+ I think you already have a model available with the dragonkin monsters in the Deep Wells
+ Flexible for all sorts of RP, not stereotyped, good or evil
+ Could be Surface or UD
15) Nymph
https://forgottenrealms.fandom.com/wiki/Nymph
+ Gives a non-evil or non-monstrous 5% option
+ Fits well with the wilderness theme of much of the surface