The TR data set is really strong on workout-specific data (Power, HR, cadence data etc.), but weak on “big picture” type data.
So, why don’t TR send out a survey to all users to aim to collect such data as:
- How many years they’ve been cycling.
- What their cycling background is (road, TT, MTB etc.).
- What other training they carry out, plus the volume for each discipline.
- Typical diet (if on LCHF etc.).
…
If enough data is collected, some useful trends might start to emerge (when tied with the workout data)… experienced cyclists respond better to plans x/y/z. Those cyclists on a LCHF diet are getting more gains from certain plans. If you’ve got a big road cycling background certain plans aren’t very effective etc…
No-one likes completing surveys, but if there’s a decent prize at stake (win a smart trainer, or a free year TR subscription) then enough people are likely to complete it 
Based on this data, TR could implement a new “Plan Builder” option into the website; so instead of selecting an off-the-shelf plan, you’d complete a series of questions (required phase / available training hours per week / training experience & background / diet etc.) that would generate a custom plan for you… all based on what the data has shown as been most effective for you.
As time goes on this “big picture” data set would become more and more comprehensive (based on the data collected in the “Plan Builder”), leading to better custom plans.