1. Nobody after Chazal, or perhaps the Geonim,can create a new halacha for all of Am Yisrael. However, the mara d’atra can create a takkana for his community. Moreover, obligatory minahim can spring up from the people. The classic examples are not eating kitniyot (which some Rishonim opposed) or gebruchts (which the Gra strongly opposed) during Pesach.
2. The Shela HaKadosh mentions that some had a minhag to only speak Hebrew on Shabbat. However, it never caught on. Perhaps because people’s conversational Hebrew was not good.
3. Regarding umbrellas, according to the Chatam Sofer (Responsa OC 72 – http://www.hebrewbooks.org/pdfpager.aspx?req=14663&st=&pgnum=54) it is not even an isur d’rabbanan. The Biur Halacha (315,7 d”h tefach) says:
“????? ???? ???? ????? ?????? ????? ???? ???? ???? ??? ???? ???? ?? ?????? ????? ?????? ???? ?? ?????? ?? ???? ?? ??? ????? ????? ???? ??????? ??? ?????? ?? ??????? ?????? ??”? ?????? ???? ?? ???? ??????? ??? ??? ?????? ??? ????? ?????”.
The Chazon Ish says that it is tikkun man and avsha milta (which I do not understand as that generally refers to noise) and adds that even in our time there is the power to make decrees for all of Am Yisrael (which is also very puzzling as there has no been a Sanhedrin for over 1,500 years).
However, the universal minahg is not to use it on Shabbat or Yom Tov (interestingly, certain types of hats which almost definitely have the din of tents are allowed because of minhag – Mishna Berura 301,151-152).