Let's talk about Phase Rush...
"I always thought Phase Rush sucked on Mordekaiser, because he couldn't activate it properly... I only now realized I actually was activating it, but it's so weak I couldn't see any difference in speed..."
Let's start with a simple question: Was Stormraider's Surge OP? I don't remember many people using it, aside from Mordekaiser players, so why is Phase Rush so much weaker? Phase Rush - 15-40% MS buff, 65% slow resistance. SRS - 40% MS buff, 75% slow resistance. This is a huge difference. During lane phase Phase Rush is so weak, it doesn't do anything to help any champ that needs it. It's not even picked by Mordekaiser - a champion who worked great with SRS. Phase rush is barely picked by him, and when it is - it has an awful win %. Even Fleet Footwork is better on Mordekaiser... And speaking of Fleet Footwork - why is it allowed to have 30% MS boost on all levels, on top of the powerful heal? And don't you say the slow resistance is better than the heal - it's not. Slows aren't that big of a deal, ever - not when 80% of champs have some hard CC. The only time Phase Rush is useful is when fighting someone like Nasus - and even then taking it feels like a waste, because the speed burst is so irrelevant.
Would making Phase Rush as strong as SRS was really break the game? I doubt it would, I even doubt many people would use it even after the buff... But it would help champions like Mordekaiser, who is struggling greatly (On toplane) ever since the new runes were introduced as not a single one really works for him. Buffing Phase Rush could potentially what he needs untill his VGU rolls out (So who the hell knows when).