Today, some brainstorming about tweaks I would experiment with, were I on the dev team, with my usual disclaimer.
My first experiment would be trying slide as a subset of dodge, where holding the dodge button initiates a slide after performing the dodge. As I recall, both moves worked on a similar 4-way axis and didn't do much differently with the camera, so assuming I'm not forgetting something, it could even be inexpensive. And since you could activate bullet-time in both dodge and the slide, whereas the current bullet-time slide was significantly harder to initiate (LB+LT, ugh!), I think it would have made the controls easier to grasp and less of an obstacle to mastery.
I'm also curious what it would have done to reduce the rate of energy drain on the slide, or remove the drain completely. I never had the impression that the move made you overpowered because I was often hit while sliding, and I'd be fine with making enemies more accurate if it meant I could experience the joy of zipping around more easily. But energy cost is usually so easy such an easy asset to tweak that I suspect the dev team had good reasons for making it expensive.
Another element I disliked was how it forced you into bullet time. Minimally, I should be allowed to deactivate it as I normally would. I don't always need it or want my energy drained, and if it led to my death anyway, it's the kind of consequence I'd blame myself for, not the designers. That's a good thing.
But I'm also curious what it would have been like to just have the traditional concept of shields in shooters, and just make using things like slides, bullet time, and melee attacks reduce your shields. It's a reckless thought, but if these actions brought you closer to a base vulnerable state, it might be an interesting baseline risk / reward system that keeps the power to reap the rewards of expertise (and pain without it) firmly in player hands. Again, I wonder if something like this was tried during development.
If the designers wanted to penalize you for dying and affecting your weapons, there probably would have been better ways to handle it than taking things from the player. In fact, you could probably have nigh-identical consequence without ever triggering anger by making it an addition-based system instead of a subtraction-based one.
For example, make it so you can achieve rank "9" with each weapon, but if you stay alive long enough, your character gains 1 bonus rank (up to 10). Assuming weapons in the released go to "10", you can see where I'm going with this. Rather than letting players get to "10" and stealing their rank back to "9", let them get to "9" and work to achieve a bonus associated with their skill. Death wipes the bonus, but players don't feel as though something was stolen from them by the designers. The game impact is essentially identical, but the sense of frustration would be greatly reduced.
That's it, off the top of my head. The story was forgettable but I enjoyed Vanquish for its moments of kinesthetic joy, and was interested in its untapped potential.