Since Sett is gonna get nerfed, here's how I would do it

Sett's main problem is his W's damage. Being able to deal up to 1k + true damage with only a Titanic Hydra and everything else being pure tank items is kinda nuts, as everyone obviously agrees.

To balance his kit, I'd simply power balance it and remove some power from W from the earlygame and shift it to scale slower

Example:

• Haymaker: Base Damage Values reduced to 60 / 80 / 100 / 120 / 140 (+60% Total AD) from 90 / 125 / 160 / 195 / 230; Grit Damage Scaling values now set to 10% / 60 Bonus AD of expended Grit instead of 25% + 10% / 100 Bonus AD

Or just anything similar to this. Reduce the W base values and increase Scaling values for late game so his damage doesn't rock as much earlygame, Introduce more scaling for his W so he is forced to build any second item with some AD Values, be it Steraks, black cleaver or even Deaths Dance, so he doesn't just rush Hydra then goes full tank.

This might not be exactly it, but if I could suggest how to address his current Power, this is how I would. Not a perfect solution, but it would IMO work.

TL;DR Shift the Power from W to weaken earlygame and make lategame more important.