What Rylai's Should Be Next Season
It shouldn't be a mana or CDR option.
You want mana? You get:

You want CDR? You get:

It should be a tanky option with slowing power. But what could possibly be the build path and stats?
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Stats: 550 Health, 65 AP. Costs 3000 gold. Passive: Spells slow enemies based on slow type. Single Target: 35% for 1 second. Area of Effect: 30% for 0.75 seconds. On Attack Effect: 40% for 1 second. (Jax W, Morde Q, ect.) Damage Over Time: 20% for 1 second. Multihit: 30% for 1 second.
Basically it's a tankier option with less damage but also slows less because of people may still buy it for a permanent slow (which is pretty toxic).
EDIT: If it was to have CDR:
, 65 AP, 450 HP, 10% CDR.
: 450 AP - COUNT ME THE FCK IN.