Would be cool if additional mobile OS themes were added at some point.
Really cool, but maybe slightly difficult due to the primary non-touch nature would be KaiOS:
https://developer.kaiostech.com/design-guide/ui-component
(KaiOS is based on FirefoxOS and used in Smart Feature Phones. Several hundred million users already).
Nice would be also Ubuntu Touch, which got “revived” by Ubports:
https://docs.ubuntu.com/phone/en/apps/design/
http://docs.ubports.com/projects/icons/en/latest/
And last but not least there is another cool one called Plasma Mobile, which uses the Kirigami Design guidelines:
https://www.plasma-mobile.org/
Thanks for considering 