I admit that I'm not a very passionate of a player, when it comes to Arnor; nor am I really versed with balance and gameplay concerning the faction. Only, in order to give a minor contribution about what caters for the conceptual side of the proposal, I deem the topic absolutely worth being explored thoroughly or, as you assessed, being given more prominence in the holistic design of the spellbook.
The long life of the Númenóreans is a prime theme in the canons, being their longevity the gift that the Valar bestowed upon the Edain, as foremost testimony of friendship and as the noblest kind of reward, after the Edain had endeavoured a lot for the good cause and contributed to put an end to Morgoth's ruthless dominance. The Archangels therefore sent their herald, Eönwë, among them, whose holy knowledge allowed those valiant Men to achieve a long lifespan and to gather power that was unknown to the other lesser lineages of mortals, thus making them mighty and almost as wise as the Eldar of Aman. But the greatest gift of the Powers was undoubtedly the raising of Númenor from the depth of the sea, in which those blessed Men could build their strength in peace, far from the troubles of Middle-earth (the island was closer to Aman than it was to the mortal continent), just to become an imperial superpower in the following centuries.
Henceforth, I definitely favour additional focus on longevity and bloodline, which is also the most evocative legacy of that grand past, resisting still in the grey Third Age of Arda. And pardon my long excursus about the lore of the Second Age/Númenor, but those pages of the Silmarillion are really wonderful to read and extremely profound, as you get to discover most of the founding causes of the misadventures of Men in the years after and the spirit of their doing in general. Passages that always win my heart, beside the lore of the Eldar in the War of the Jewels or during their staying in the Undying Lands, to which my soul belongs.
Tales tell that Elros was the oldest Man ever, having lived for 500 years and reigned for more than 400. Legends and memories of when Men were much akin to the immortal kind as never before in the annals of Arda.