Expanding the Northern Reaches: Giving Ostland their own Elf community (and an alternative for Halflings)
by Marc SaindonGaz7 (p.9) initially limits Elf or Halfling in the character creation, so if your character intends to play one of these, he'll be an outsider and going "13th Warrior" might not be the campaign you want. A quick fix is having a remnant of an Icevale Elf community miraculously surviving outside of the Hollow World due to some interventions by the Vanir Freyr (considered to be the 'King of the Alfar' in norse myth), and thus managed to develop a small community in Ostland on Kalslo island. They might have played a crucial part in stirring human settlers towards the worship of the Vanir instead of the Aesir.
The Kalslo Alfar (or just 'Alfar') have little or no connections culturally with the clans of Wendar or Alfheim (which maybe should be renamed Canolbarth to avoid confusion with the norse myth realm...). They would have a rivalry with the Rot Dwarves (Modrigswerg), and in human mindset, this would be re-interpretated as a struggle between the "Light Elves" (Ljosalfar) and the "Dark Elves" (Dokkalfar, despite them not being Elves); yes this is intentionally done to exclude Drow as a solution to a continuity problem.
Ostland Elves would use throwing axes and javelines instead of bows (despite HW Icevale Elf being good archers), and be a bit stockier than the vanilla elves of Mystara (see the Icevale Elf entries in the Hollow World boxset). They're not the BECMI Elf Mage/Fighter, but a Warrior-Elf class that xp faster. In 5e terms, this would mean that Alfar Wizards and multi-class magic users are rarer, and specialists like the Treekeepers don't exist, and probably the "supernatural talent" among the Alfar tends to lean toward more among Druids and Rangers.
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As for Halflings, two options:
1) Gnomes fill in the niche that the Hin occupy in the Known World, and despite having lost their homeland of Falun, the Forest Gnomes diaspora is still a common sight in Vestland and the Northern Reaches.
The 'Hidden Folk' are retconned as Qeodhar's indigenous population, now a minority in their own homeland. They play the same role of Halflings, although don't have hairy feet, but a calf tail (which is rude to speak about), and are rumored to have hollowed backs (which is only rumors). They probably herd a lot of sheep and work with sheep related products (wool, cheese).
(art from: https://www.deviantart.com/ejve/art/Ice-Elf-615712520)