This won't happen, unfortunately. It would be easier to make more sentences than it would to design a system (if a system can even be designed) that would be able to accurate replace parts of a sentence with other parts and make sure it makes grammatical and semantic sentence.
Even if I spent time applying categories to words (all these words are people, these are colors, these are feelings, these are actions that affect other people, etc.), you'd still come up with problematic sentences right out of the gate, even if you assume perfect grammar match up (and of course, the English translations would be useless). All of a sudden, the sentence "the father talked to his son" becomes "the father punched his son."
My guess is that other apps may have something like this (maybe even for something simple, where colors are replaced), but I will not add anything to renshuu unless I can be sure that it is creating/using natural, accurate Japanese.
This is the same reason why the sentence questions on renshuu pro do not do multiple choice for words that go into a sentence. It's something I would *love* to make, but it simply isn't feasible.
