Honestly, I don't think it needs to be as complicated as Jagex is trying to make it. It's pretty simple, just increase the effect that your agility level has on run energy usage by a little bit every 10 levels (and at 99), so that we can run just a bit longer. We don't need infinite run, we don't need to adjust graceful. Just make the agility level impact gameplay a little bit more.

Also, might be cool to have some extra training areas/methods locked behind agility requirements. Nothing gamebreaking, just extra spawns for often camped things like sand crabs, maybe an extra farming patch, stuff that feels like a nice prize for getting your agility up.