I thought Arboreal was used for forested regions. Seems Nemoral can also be used. Would avoid the confusion Arboreal ~ Boreal.
Not quite - 'arboreal' is used to describe something related to trees and 'nemoral' describes something related to woods or groves, but 'boreal' describes something related to, or coming from 'the North' (as in 'Aurora Borealis'). It's usually used in the context of Boreal forests (also known as 'Taiga' - and are characterised by coniferous forests) or Boreal climate (characterised by long winters and short, cool or mild summers). A 'Boreal' world might be somewhat similar in climate and appearance to Canada, Scandinavia or northern Russia.
I have occasionally seen 'Nemoral' used to describe temperate forest biomes but that terminology seems to be in decline.
Also - regarding adding new types of habitable worlds - I'd personally be all for it but I'm slightly worried that Paradox may have boxed themselves into a corner with their current 'ring' system of planet colonisability and their desire for a game with equal starts - it somewhat limits what can be added.