Let me turn on or off ERG mode whenever I want, in a Free Ride, not in a workout. Let me set the power target to whatever I want. from a UI point of view, this is not a very complex feature to design.
Rouvy at least has a Power Control workout (which is better but still not as good as ERG as a button whenever I want to toggle it).
Searching the forum, it looks like various versions of this feature request have existed for more than 10 years.
the UX on Zwift and Rouvy is pathetic.
Gears exist.
Welcome to the forum.
I think it’s a reasonable feature request, that so far lacks much support so nothing has happened. It’s certainly not a bad idea that I would not want implemented. I don’t see it as a UI issue either, it’s a feature that some people would want. I suspect most people who want to use ERG mode in a free ride use a cycling computer or Garmin watch to control the trainer’s resistance while using Zwift, so they employ a workaround to get past the application’s limitations. For this feature to get any priority it would be helpful to hear from more people who would use it.
This may not satisfy exactly what you want, but I’ve found the Elie Zona device very handy to do somewhat what you describe.
I think it’s a weird feature but, I too at times just want to hold a specific power, and being able to do that on the fly would be really nice.
You could probably do this via a Garmin Edge bike computer.
I used to see a few folks do this with the Constance robopacer group, and have also heard of some ride-leaders doing the same when leading a Zwift group ride. You can pick when people are doing it, because their power doesn’t have any fluctuation.
Resistance is controlled by your Garmin, rather than Zwift. Doing it this way will get you what you want faster.
You can turn trainer difficulty and just keep constant gear and cadence but why would you, it’s boring as hell. Personally I’d prefer if we could do workouts in sim mode so you’d need to continously shift to maintain power targets
What would also be handy is what I used to setup in PerfPro Studio, interval blocks with set gradients, example:
4x15min at 7% gradient
The recovery blocks in between were easy 2%.
You used gears and cadence to hold the desired power. I found those workouts really effective. Somewhat similar to your idea. When these intervals got easier I made work blocks a bit longer, the recovery a bit shorter and/or made the gradient steeper. Eventually the 4x15min gets closer to a 1x60min.
The other thing I do these days is treat the ADZ segments as intervals trying to hold a set average power for each sector, using the segment indicator HUD as a guide. It isn’t as easy as it sounds because the gradients are always changing and sometimes the gears you have might not be quite right (one gear too small, another too high for example). This is close to your idea.
Or alternatively you can watch heart rate to make sure you don’t go above a certain limit. I do that sometimes.
I find that less boring than long ERG mode intervals, but someone who actually likes ERG mode probably won’t be satisfied by it.
TPvirtual has this available, they read the zwift forums and implement all the good ideas that get ignored here. @G_80_M3
I understand I love ERG mode that puts me in the minority. But from an app development point of view, expanding ERG mode is trivial. In my opinion, ERG mode is what makes indoor cycling great because there is no equivalent in outdoor riding. There are plenty of times I don’t want to deal with gears, I just want to put my head down and pedal.
At present, I have to create a custom workout with zones of predetermined duration and intensity. That’s annoying. I’ve got a bunch of custom workouts for just that, all the way up to 6 hours (in 10 minute blocks, because of the annoying decision to award XP only after block completion, not on actual minutes of effort). But the super aggravating bit of this approach is, if I’m in a block set to 100 W, the Zwift +/- buttons only let me whammy up or down by 25%. so 125 W is the highest I can do. Honestly, WHY?? Why put this limit in at all? Especially in a custom workout.
Rouvy has Power Control workouts, which is great. but also limited by UX. In Rouvy, if you want to jump from 100 W to 200 W you can have click the little + button 20 times. They are too lazy to implement a UI to snap to your power zones or recently used targets.
As I said, the UX of both Zwift and Rouvy is very lazy and frustrating. I mention Rouvy here since Zwift now owns it, which I assume means both Rouvy and Zwift will just circle the drain while the executives try to wring out every dollar of revenue while reducing development costs.
I don’t question your desire for ERG mode wherever you want it - you like what you like and that’s fine. But to get a feature added to Zwift, it needs to be something that a lot of people know they want, or that Zwift thinks a lot of people will enjoy using. That isn’t circling the drain - it’s spending money on things that will provide a return on the investment. I don’t know if this feature would or would not do that, with basically a trickle of interest in it over many years. Although Zwift acquired Rouvy, this forum is not an effective place to influence Rouvy’s development plans. For that you would need to approach them on social media like on their Facebook group, and see if that sparks any interest. My tip for doing that is to avoid telling them that they suck and instead tell them you love the product but need this thing and explain why you think it would be useful.
Technically setting TD to 0% isn’t the same either, not by a long shot.
ERG does the job for me of “changing gears”, and for people on manual shifting trainers means it can probably also save a lot of life for all components combined if they used ERG more.
I did a climb portal last night on my Tacx set at I think 90% TD and was almost disgusted by how much I was shifting lol (I forget what climb it was, lots of ups and downs!)
Honestly there’s just no real reason as to the why Zwift can’t do this. It’s already there in workout mode, and grade mode… it’s already sending the same data to a trainer…
It’s just allowing us to force the grade number. That’s all ERG is by data.
Of course yes a bike computer or watch can do it, but… zwift can / could too.
As a training tool I don’t feel like this is asking for very much. It’s almost like asking for the most basic feature it can offer actually.
Here’s a tempo ride with TD 0% up the Alpe a few days ago. I don’t get ERG mode’s help at maintaining steady power, but I still do the steady power. The body doesn’t know the difference.
But if you go to stand… you have to shift to maintain it.
That’s where the whole 0% TD thing breaks down and the purpose of ERG exists.
Plus to be able to do that we have to go into settings menus, and we have to remember visually where our slider is for when we want to go back since we can’t do something as insane UI wise like right clicking a slider to input a number, that would be wacky, what kind of monster would do that 
Yes absolutely, and I did stand a few times during that session, though they are barely visible on the graph. Anyway I am absolutely not opposed to this feature request. I wouldn’t use it but that’s OK. The real question is how many people would use it. I hope they all drop a comment here. Maybe because the workaround of using a cycling computer, smart watch, or smartphone app to control trainer resistance is so easy and effective, they stop caring after they get that sorted out. There are a lot of missing features in Zwift for which the workarounds are pretty bad but I’m not sure this is one of them.
If you want to hold wattage setpoints I personally use my BeatBiker app in virtual gearing mode for this. I think other apps offer similar features.