At least in my mind I am not disagreeing with you 
Find yourself an original Tacx iMagic from the 90s. They had a constant-HR training mode - resistance going up and down to keep you at a set HR level, which you could program in workouts.
Tried it a few times, it really sucked.
I’d love to have the ability to do a workout where the power is controlled in order to keep HR within a specific zone (Z2).
Did this feature request die a death or is something (please) being considered?
My Schwinn Velodyne trainer that I bought in 1989 has that function. I never use it, though.
For one thing, even when power drops to near zero after you drift out of range, it takes too long for HR to come back down again - at lower intensities, where you might be tempted to “clamp” things, your legs, breathing, etc , recover much more rapidly than HR.
The opposite problem exists at the start of workouts. Increasing the power is what drives HR up, but due to the lag of the CV system, it takes far too long to get into the “zone”.
Finally, all of this is completely unnecessary, and in fact even counterproductive, at least if you want to be a good all-around cyclist (vs. TTer or triathlete). Power is highly variable in the real world; too much time being a “trainer drone” just turns you into one of those one-speed bike riders that loses the wheel after each corner, on small rises, or any other time there is a surge in the pack.
Yeah, OP request might be overly precise, I’d be satisfied with “keep me in requested HR zone” as well, implemented in any simplistic way a la past 5min HR level to decide power level for next 5min.
For me it is actually other way around – initially HR goes higher than expected and then settles down in 20-30min, for 2h low Z2 cardiac drift might be even around -5%. As I understand, this is not my personal trait, there was topic while back about negative drift.
Regarding being counterproductive, main goal would be to use it for easy days between 2-3 harder key workouts within week to maintain consistency
It’s probably already been mentioned but TrainerDay has exactly that functionality and it costs about £3 per month!
I’ve been using it for a while, I like it. It works.
To be honest, for me personally it does not matter much, I already have my favorite endurance TR workouts for which I know keeping me in expected range. And if it doesn’t, then something else is wrong (sneaking illness, long term fatigue, etc).
Actually, it makes me consider that I would not even use this feature because those workouts are like checkpoint to see if EF is as expected or improving.
Two words: erg mode.
Erg mode doesn’t help help when it doesn’t account for HR Drift. Like @sven says. The Cardiac drift being there is the monkey wrench in that equation.
For Z2 workouts I run my trainer in ERG mode controlled from my ELEMNT BOLT, I have a page on the BOLT configured to show showing Power target, 3sec Power, cadence, HR and total time if my HR gets a bit high I can just use the buttons on the computer top drop the target wattage 5-10w. This is important for me because I tend to do my Z2 work immediately after my evening meal, and straying into Z3 HR has me tasting my dinner again 
If you’re getting HR drifting out of Z2 when in ERG mode your Z2 power target was probably too high to begin with.
What even is “perfect Z2” training anyway? Its all a continuum, there is no magic bullet.
Yes… But most that request for these basically wants “hands free” way to do it.
Except that cardiac drift doesn’t matter.
Want to do some good endurance training w/o having to think about it? Toss the HR monitor, pick a power, put your trainer in erg mode, and fire up a movie.
This. Everybody’s looking for a shortcut, and following the latest Pied Piper (Seiler, ISM…I wonder who it will be next year?).
What about doing that for VO2max short intervals, like 30/15?
Start with a predefined value and then let AI adjust the intervals duration or power to get you heartrate to a high target (something like 92/94% of max HR), and enough rest to go down to a low target (90%).
As you fatigue power will go down, but you will be able to stay in the target HR zone.
Using a bit of AI to reach the targets, not simply looking at the instant HR.
If you don’t mind to share, what would be your approach for a base period?
Asking because it looks like you aren’t a huge fan of z2/endurance as a driver, or I’m getting it all wrong.
What is “base” training?
The out-of-competition-season program described in slide 32 of this presentation worked pretty well:
Really, though, it depends on how much time you have to train, who you are, etc.
For a year intensity decrease and volume increase worked really well for me
→ 
TSS remained roughly same (28k → 29k). No noteworthy FTP increase, but other aspects I care about (TTE for 4-12h, ability to push late in ride), improved considerably.
Although, to be fair, for now gains have plateaued. It is time to return more frequent higher intensity stuff. Luckily new revised TR plans seem nice change, exactly what I would have done for myself.
Point being: at right time for right period, Z2-centric approach can work very well. And when somebody wants to focus on it, why not improve TR workout player to help achieve that? Plus I like @ChrisDe idea for VO2max as well.
Sure, but this often gets conflated or attributed to the foo around Z2.
In what way? See current thread.
Sure, can’t argue here. And it is possible my development over that year could have been similar with usual TR approach as well, because it was my 3rd year of structured training and it was simply time for that boost 
“Work well” depends on a lot of things:
- How do you measure success?
- Was it that your current training approach is “wrong” or did you just need a novel stimulus?
Don’t take this as me dismissing what you wrote: Your experience that doing more Z2 and less intensity will manifest itself more in TTE gains and less in FTP gains jives with my experience with polarized plans.
I have followed training plans that had an intentional mix of polarized blocks and base blocks that include sweet spot for three seasons. Knowing your goals and knowing how you judge success is crucial (and something you mentioned explicitly in your post), though.
Rather than religiously subscribing to one particular training approach, I try to collect arrows in my quiver and stick to the basics such as consistency, periodization and progressive overload.
Plateauing could be due to you reaching the limit with that particular style of training block. You could alternate your focus where in one year you focus on broadening your base and then building the height of your “power tower” the next.
TR’s offerings have become better and better. The fact that I can pick between three different base approaches now is great (although I have never tried traditional base), pick training time limits and opt for Master’s Plans (which could be a great way to deal with fatigue if you have been too ambitious or there is additional life stress).
Yes, and it is often hard to suss that out, because any reasonable structured training plan will make you more fit. And you can’t make the experiment where an alternate version of you follows a different training plan.
Trying new things — even just to shake things up and not repeat the same recipe for the 3rd or 4th time — is good.