Quick preface: thanks, Riot for listening to us on Zac's sub-380 walking animation and voice pitch being too obnoxious. It's really appreciated!
Since it appears Riot is deadset on keeping Slingshot the same, diminished power level and Zac quite generally sucks on the PBE, I had a couple ideas that would feel really nice to play with as Zac, especially a skilled user.
1) Give him his March 29, 2013 (release) numbers on his blobs again. This means decreasing the time gap between the period globs land and when Zac can actually pick them up, benefiting those with quick reactions and pathing while encouraging quick thinking. Keep the distance at which they fly the same.
2) Put Zac's ultimate on half the cooldown if he channels but does **not** use the reactivation. This enables him to use it tactically to apply a slow in a tight corridor and benefit from the reduced cooldown by knowledgeably selecting the correct of the 3 options Zac's ultimate would have. Also enables you to use it to soak/nullify cc's.
3) Further accentuating the idea of variations of Zac's ultimate, lower the distance at which Zac knocks back enemies with his instant cast variation, but grant him temporary bonus size and maximum/current health to create a barrier of sorts between target(s) he's isolated and the rest of the enemy team. Or perhaps his next 3 autoattacks after the instant cast ultimate slow for 20-30% and drop a blob to compensate Zac for the lost blob generation of his new ultimate. (I like the latter idea better due to Gargoyle Stoneplate's redundancy)
4) Picking up a blob grants Zac a quick burst of temporary movement speed. (this coupled with #1 would be my ideal fantasy). This allows Zac to continue his flexible, fun playstyle while maintaining his ability to stay on a target with his ultimate stickiness+slingshot knockup removed/nerfed respectively. Perhaps lower health regained per blob from 4% to 3% max HP.
I hope you like these suggestions. :)