So... this is just a funny build to try out and cause chaos.
Also remember you're a support, so keep buying wards.

EDIT: Remember to shout "CHOO CHOO" out loud, when activating your W and Shurelya's.(and Ghost, if equipped)
EDIT2: It is possible to buy a Phantom Dancer, or pick Ghost instead of Exhaust for even more fun.

So yeah, I was just playing around with Blitzcrank for a bit, and found out that when buying items with some movement speed, you really mess some teams up. You do need some guts to hold your ground, when a Blitzcrank is running towards you with 600 movement speed.

Mark of Resilience:
Makes you somewhat tanky. Ideal if you got an opponent against you who is aggresive, or is poking you.

Seal of Replenishment:
If playing somewhat mana-conservative, you wont run out of mana, and you can grab more often.

Glyph of Warding:
Makes you somewhat tanky. Ideal if you got an opponent against you who is aggresive, or is poking you.

Quintessence of Potency:
Gives you some additional damage on your grab and ultimate. This is not that usefull though, I am looking for a decent replacement.

Pros:
Sick movement speed
Tanky (mana barrier)
You can make the difference in teamfights
You can deal with enemies who are low on health, and not too dangerous.

Cons:
Requires skills with Rocket Grab
Damage output as a support: LOW
Manahungry if not played properly