What if somebody with a channel like Anivia just sits in nexus for a bit of time with ult up to charge mana? She'll never oom and will just build up stacks really quickly...

Did a bit of math:
Assuming you mana do spend mana every 3 seconds, it would take the following times to fully stack:

New Tear:
750 mana / 4 mana = 187,5
187,5 * 3 seconds = 562,5 seconds = 9,375 minutes

New Archangels:
750 mana / 5 mana = 150
150 * 3 seconds = 450 seconds = 7,5 minutes

During this calculations, a new question arose for me: If you already stacked your Tear, does Archangel's instantly turn into Seraph's Embrace?

I don't agree with this at all. Putting another champion passive as an on use item, Ryze with this item is going to be insane. Even more so because late game he has no mana issues what so ever.

Xyph, do you see this potentially creating new subsets of existing champions? Ie: Tank Karthus who gets things like this item, frozen heart, banshee's, etc--ie: lower damage output than regular Karthus, but about 3-4 times harder to take down.

Side note: Tank Kassadin in Dominion is going to love these changes (less cost, thus slightly better early game and even more beyond godlike late-game).

I'm really excited to see what if anything you've done to Lich Bane.

i dont like how you lowered the mana regen on it. and lowered the flat mana. and maximum mana charge.

It occurs to me that unlike current AA, the bonus mana on seraphs isn't unique.
So ryze can stack 5 of these for a 5000 mana bonus (you'd probably want a void staff too though).
Ending up with 6240 mana, and 487 ap (300 from item, 187 from the 3% of your mana = ap passive).
Also a shield protecting up to 1560 damage.
Daaaaaayum