I am almost sure that some of the things I'm gonna write down have already been touched in the past. Anyway I'm gonna post this post.
In my building experience with LEGO I've always found the way to build, somehow, what I had in mind. Sometimes the result was not satisfactory because I'm not good enough, sometimes I don't have enough of the parts I need.
BUT, in my opinion, there are some pieces that are missing, in the sense that LEGO has never realized them and they are very very common in medieval architecture.
I was in Italy (my homeland) during the holidays and I had the opportunity to travel a bit in some very nice and very well conserved medieval villages and cities (Gubbio, Assisi, Perugia, Spello) and of course travelling is also a source of inspiration for building.
So, among the pieces that are missing, there are for sure the lancet arch (in Italian "arco a sesto acuto") and the window called in Italian "bifora" (I don't know why but I cannot find the english translation of this word).
So the lancet arch:
And the bifora:
I personally think that for the lancet arch we would need a dedicated piece and I would be really pleased if LEGO is lessening at me, even if I'm convinced that this request has been done in the past.
About the "bifora" I don't think we need a dedicated part. What we are missing is just the double arch that could be done from the ones that are widely produced:
The fact is that the double arch should have only one stud in the middle
, which has already been done in this rare piece.
If it has been done for slopes, why not for arch? If they will start to produce both pieces our MOCs could be enriched very very much!