As someone who used to log lifting in plain text files (copied from a notebook) I think this is a really good idea.

To avoid all of the comments from people asking about GPS tracking, I think you should make it clear that it's primarily for strength training. The format is clearly oriented around that.

An addition that could make it more useful for recording cardio workouts like swimming/track/indoor cycling would be the concept of rest intervals. For example, it's common to prescribe a part of a swimming workout in a format like this:

  4 x 100 at 1:20

Meaning you have 1:20 to complete each 100m rep, where any extra time is rest. You could also add extra fields for recording the actual time taken to complete rach rep. E.g:

  #Unit: m
  #TimeFormat: M:S
  
  4r 100 @1:20
    1:05, 1:07, 1:11, 1:06