Pantheon is actually balanced AROUND enemies having some time to predict his location, as he can easily burst down squishy characters that he catches EVEN IF THE ULT MISSES THEM.
As such, you'd either need to nerf his Q/W/E damage enough that targets escape him easily when he doesn't hit his ult, or accept the warning timer.
Personally I find it a fantastic tool once you get used to it -- it's less an initiation tool and more a follow-up to existing initiation (your Amumu ulted and your Support Zyra has ult ready to go? Dive in for the finish!). More than either of those it's a VERY powerful zoning tool. If you're going to gank a lane, ult in behind the target to force them to make a really tough decision: escape through your ult to the tower, probably taking the full ult damage and risking Pantheon diving them, or run to the side, possibly into your teammates, and probably getting flanked between you and them.
It can easily win teamfights: not due to its DAMAGE, but due to the zoning potential and fear the ability can inspire in a foe.