Look it's all just up to personal opinion at this point. Blitz does have what we're talking about, but self-imposed buff/debuffs are different to those imposed by someone else. Blitz was actually given that as a nerf last year because his old one lasted 8 seconds with no drawbacks. Sure the boost it provided was significantly less, but his old method offered less counterplay.
A window of opportunity if fine on yourself, but debuffs to allies and buffs to enemies is something I personally can't get behind. The debuff would have to come to you, not your ally. I've seen ideas posted before of a parasite themed Void champ, and I've seen a bunch of ideas and interpretations of how a kit like that could work. Things like 'providing ally X bonus damage, but reducing your damage by X" or "mitigate X percent of damage taken by target ally to you". Those are buffs to your ally with a drawback, but in X seconds time, they will still be good to fight and you will be the one that can't. You wouldn't want an effect that can take your carry out of the game.
Anyway that's just me.