AP Item Pass and its Distribution of Mana

With the new mana items giving 500 mana (100 more than morellonomicon) and them probably being the first item mages will buy, is mana being relegated to a "buy once and forget" state? I feel like this might shift some powerspikes around, especially for mages that normally go 2 mana items like Zilean and Anivia.

Should mana be frontloaded onto the first item you buy, or should it be spread out across your build? This is a question that will affect the health of the game, and I feel like maybe a small portion of the mana should be transferred to some of the more glass-cannon AP items such as Twin Shadows without making the item inefficient for manaless champions.