Cant you make they look differently more for faster or slow ?
They do, the leg and arm speed changes the faster you run like it does in real life.
Not sure what other changes you would like?
It looks too controlled, too perfect. A real runner loses form when he (or she) is pushing the limits, be it distance or speed. Yesterday I pushed myself to set some decent first 5k and 10k times in Zwift, suffering through the second half. The avatar looked as if he was doing an easy warmup the whole time.
For better immersion it would definitely help if the avatar would start losing a bit of form depending on my effort. It would take the same data and logic as the training programs are using to determine what I’m capable of. Plus a bit of extra animation.
But how would it know you’re losing form?
I think your expectations are a little high.
You’ll be asking for mid on your legs when running off road next 
If the training schedules can determine what it takes for me to work hard, then that same logic can be used for the avatar.
I agree that this is not 100% accurate, but it might remove some of the plasticity and make the avatar a bit more human and likeable. Common thing in gaming. Maybe add a flag in the settings for people who don’t want it.
Don’t you find though that 2 days are never the same with running? I run halves at a flat pace. One day I’ll bear it the next I’ll suffer with the only metric to show this is my heart rate.
Other than Zwift knowing my max hr I’m not sure how it could tell how my form is.
I like the idea you propose but not sure how achievable it is. If my avatar sweated as much as me there’d be a pool of water on screen behind me.
Just noticed that for cyclists this is already a thing. While running up a steep slope, I passed a cyclist who was standing on the paddles. A quick Google search showed the same for sprinting: rider off the saddle, bike swinging between the legs. That’s exactly what I mean.
A runner will lean further forward and maybe hunch just a bit when running uphill. Lean back downhill. Also lean forward more when really pushing it in the red training zone.
I also noticed a cyclist reaching down for a water bottle and taking a sip while riding slowly. This is off course not a sensor driven copy of what the actual rider is doing, but it does communicate to the player: “wow, we did a bit of work on the difficult part there. Let’s enjoy the easier part now”.
A runner could maybe wipe his forehead or stretch his arms down for a couple of steps.