Roles Revamped
Teamfight Tactics Set 15: K.O. Coliseum is making some big changes to how units are categorized, and with it comes updates to how they'll now gain mana. These system changes are perhaps in part related to preparation for Set 16, which Mortdog has previously suggested will change the way we play TFT. Be sure to check out our set reveal for more details on comps, units, traits, and more!
You can read the full rundown from Riot here, but otherwise we've got the TLDR for those of you who just need the quick and dirty:
Units will all be categorized into one of five main classes that each gain mana in their own unique way:
Tank
5 mana per attack
Generates mana from taking damage
More likely to be targeted (in scenarios where a unit has an equal chance to attack two units, tanks will be more likely to be targeted)
Fighter
10 mana per attack
Has 10% built-in Omnivamp
Assassin
10 mana per attack
Less likely to be attacked (in scenarios where a unit has an equal chance to attack two units, assassins will be less likely to be targeted)
Marksman
10 mana per attack
Caster
7 mana per attack
Generates 2 mana per second
Specialist
Generate resources in their own unique way
How does this affect how you play?
Ideally, this means targeting will be more consistent -- you'll have less fights decided by targeting RNG. It also means itemization will need to be more thoughtful (thankfully, we'll have you covered there). For example, casters only generate 7 mana per attack, which means attack speed will be less valuable to them than in the past. It also means that CC will be more devastating on DPS units now as they will not generate mana while stunned.

How are items being changed as a result of this?
As you may have guessed, mana generating items are being overhauled, with tears no longer granting 15 mana and are instead granting 1 additional mana regen (taking a caster from 7 per attack to 8, for example).
Items granting 5 mana regen:
Blue Buff
Items granting 2 mana regen:
Morellonomicon
Adaptive Helm (also grants 15% additional mana from all sources)
Items granting 1 mana regen:
Tear of the Goddess (component)
Spear of Shojin
Archangel's Staff
Hand of Justice
Protector's Vow