This is what really interested me as well. Sounds from above as though it will adjust the weights (e.g. using HR to normalise) so that it gets more data points without having to test too often. That would be a big plus for me.

Also then takes me on to another tangent as to what other data points it can take as input. E.g. I know my HR varies quite a bit depending on temperature, is typically lower at any given power cycling indoors vs outdoors, and to a lesser extent I see some HR variation depending on time of day. My HR also varies quite a bit depending on how rested I am, when tapered I hit higher HRs at any given pace/power. Presumably a sophisticated enough NN could pick up all those data points (it's all in the Garmin files) to predict fitness/performance better. A NN could also maybe make better use of the manually recorded data points that are captured in TP such as RPE.

Other thought is that I'm finding HRV increasingly useful for tracking non-training stress (travel, poor sleep, sickness, bad day at work, etc). Started using the HRV4Training app back in November, still learning how best to use the data but already it's apparent that in my case at least non-training stress can make the PMC largely irrelevant. E.g. in November I was pretty busy with work, spent a lot of time in airports, planes, hotels and high pressure meetings. Second half of December and YTD on the other hand have been very quiet. I've been disciplined with training, diet and sleep throughout (TSS per week, weight and average sleep hours have stayed fairly consistent). So using PMC you'd predict similar performance now to back in November. However if I look at my Recovery Points in the HRV app, they've jumped from an average of 7.6 in November to 8.5 in the last 4 weeks. And that actually tracks far better with my training/testing numbers, which took a big dip in November but are now a bit above where I was at in September/October. Seems pretty obvious looking back that the HRV numbers picked up on the work/life stress, and that that stress had a significant impact on my performance. So I wonder if you could feed in HRV numbers to a NN model as well, and use those as a proxy for measuring non-training stress and the impact it has?

Probably letting my imagination run away with me a bit here, but I'm another one who has always found PMC to be a fairly blunt tool at best and would be very excited if something better came along.