Kassadin Riftwalk Suggestion

Riftwalk, RANGE 700 Each subsequent Riftwalk within the next 10 seconds (20 seconds from most recent damage taken from enemy champions) costs double the mana and deals bonus damage, stacking up to 4 times. if an enemy is within 525 range of him (taken from kayle's E auto range as rule of thumb) his ult gets reduced to patch 5.4 range (450).

That way people have a better chance of catching up to him (since he could often escape far too easily). But he still gets his element of surprise when engaging. A bit like how eve can be detected when stealth if she gets too close. Fitting for an assassin. Keep their distance until its time for the kill.