They were pretty solidly good guys back in the day.
I mean more in their capacity to retaliate with zero restraint. The non canon shit about them being slavers that gets brought up by edge lords aside. the House of Elros tended to have a Minbari's concept of "appropriate responses to hostility"
So time to go stop all the primitive and "evil" men from being horribly cruel to one another?
Man the North would become stupid wealthy and likely way more populated as Numenorean agriculture and tech gets traded over and negates the harsh effects of winter. If this is during the era of the good kings, then I expect there to be a lot of crossover in terms of alliances with the stark but there likely wouldn't be any marriage pacts. Lesser Numenoreans may take up settling around White Harbor which is suddenly going to become vastly wealthier and more prosperous. There'd be an exodus of small folk into the North if Bob's rebellion begins.
As for how they'd interact with Westeros..The King of Numenor is an Emperor in all but name and master of the most advanced, militarily and financially powerful entity on the planet and its got weaponry that would fodderize Others and probably even make a decent penetration into Sothoryos. These dudes are what Vlaryians wished they were and that's the other thing.
Depending on who sits on the Throne..there
will be conquest. Because even the beneficent kings still established colonies. Granted I think they'd invade the Step Stones and go after slaver's bay and likely form a very strong alliance with Braavos and House Stark...which lets be real here, is going to cause the Tyrells, the Hightowers and the Lannisters to shit a brick..and Aerys now has a superpower that renders his entire dynasty as little more than curiosities right next door.
I think the Iron Islanders and Essosi Slavers would want to raid it and the Westerosi Houses will keep on proposing marriages and maybe want to commit revenge upon rejection
The Iron Islanders would cease to exist the moment they slaughtered some Numenorean civilians and Essosi slavers might end up conquered.
TBH assuming The King of the Seas sits Robert's rebellion out..and honestly, I don't think they would. In the sense that its a dangerous civil war by the one power that can rival them, under thraldom of a tyrant..and they likely would be rather close to the Starks of Winterfell for a myriad of reasons. It's certainly possible..and assured in their more imperialistic days that they'd get involved.
At which point we see Robert Baratheon decked out in Numenorean forged armor at the trident. And It's entirely possible the army that reaches kings landing first, flies the banners of the Dire Wolf and the White tree.
Which opens up a whole can o'worms.
