I wanted another mechanic that could build up speed, but one that didn't require another button.
Introducing sprinting! By running on the ground uninterrupted, you'll continue to gain speed. It takes a bit to kick in, but if you can maintain it for long enough, you can build up some surprisingly crazy speeds. If you slow down even a bit or stop moving, you'll lose that added speed and will need to keep running to gain it again.
As you can probably imagine, this combined with Turbo (and especially in Uncle Swordsman's Story) can be really funny. I actually had to nerf Bad Guys exclusively in Turbo (your meter drains faster now) because of how broken the combination of turbo and sprinting was--you could stay invincible for the entire level with no effort!
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