New types of exercise: change or recognize tense
It would be nice if we could have two types of new exercises:
1) Change the tense of a given sentence from A to B.
2) Recognize tense (multiple choice)
These would be nice, because in the current exercises for a certain tense one can be confident that all sentences are in the tense being learned.
In reality, though, one often needs to recognize the tense and change sentences from one tense to another.
These exercises would be reasonably easy to implement, and would actually make one THINK about the differences of the tenses.
The problem with your suggestion is that Duolingo wants their learners to be able to learn the language even if they don't want to mess with grammar. So to say, they want the learners to "get a feel" of the language.
I don't necessarily agree with this idea, but using strict grammar expressions is (in my opinion) beyond this method of learning.