I think he meant a complete standalone faction, just like the other ones.
Anyway - first of all, welcome to Modding Union^^ - the team won't implement Arnor as a standalone faction for two main reasons:
1. Its gameplay would be far too similar to Gondor, therefore there wouldn't be much uniqueness and differentiation
2. Apart for the already planned 9 factions (one is yet to be released), the ET has room left for two other factions, only (due to engine limitations), so they won't waste a faction slot for a faction who's nearly identical to an already existant one
But, as DieWalkure correctly said, you can play with Arnor in their appropriate maps when you pick Gondor^^