For a lot of my activities like hiking and ski mountaineering (big days), I do not have an option to add in weight carried. I think this would be a really good metric to have on workouts to help quantify times and variations due to things like extra carried weight. Having this (or any type of customizeable field) that I can see when I look at the calendar view would be very helpful.

I would guess that you get a good number of people using TP from some of the Uphill Athlete mountaineering plans (I think the most popular training plan), and a lot of that training involves various weighted hikes.

In a similar vein (though maybe harder to implement), would be integrating that carried weight into computed TSS. I recently went through a long training program, and by the end was able to hike the same trail with a 45# pack in less time than I could hike it (same Z2 HR) in the beginning of the training. However my resultant TSS was lower. I think it should have been higher because I was way fitter to be able to carry that weight at the same speed.