Bringing the old man to a modern era (Zilean)

Zilean has had changes, and the upgrade to double bombs, while a bit buggy at times, is a step in the right direction for modernizing Zilean. It makes his gameplay a bit less binary and adds more counterplay. The problem is, adding counterplay to a one-trick pony generally results in said champion falling apart. It's why Veigar's currently in a bad spot.

Part of the issue is rewind. It's a cool concept, but outdated as an individual skill, especially since it does not affect his ultimate now.

My proposal?

1. Replace rewind entirely, but change his Q and E so that you can recast that skill within 2 seconds, but at a 50% increased mana cost (essentially replacing the mana cost of rewind). Multi-cast mechanics are more common in League now (Fiora Q, Darius R, Riven Q etc), and so adding this to Zilean while retaining the mana costs is just a step in the right direction.
   This might need to be gated to avoid abuse, and if so it should perhaps be part of his passive, which would act as a buff preventing him from using it (recently rewound), and could instead scale with level.

2. Add an extra effect to E if it is cast on the same target. For example, double casting an enemy would then immobilise them, and double-casting an ally could provide an additional attack speed buff.

3. Replace rewind with a new skill. Not sure what, but it should be a skill which can be recast within a short timeframe, and has a bonus effect if used twice on the same target. Probably utility-focused though.

The idea of this being that Zilean gains additional identity as a time master, which could translate into a champion based around the double-cast mechanic. By allowing a cast to be done on the same target, you get the option for a bigger effect on that target, but it should not outclass the option for double casting on separate targets - just add meaningful choices.

For example, you may want to slow 2 chasing/escaping enemies, or plant bombs on 2 low-hp enemies escping in different directions. But if there's a fed hypercarry, might want to double-drop the slow in order to get the immobilisation effect so your allies can focus them down quickly.

Supports should be about options - Lulu is a great example of this, and Zilean should be too, without losing his signature bomb bursts.