First, there needs to be a proposal for adding a feature (I'm not aware of one having been submitted recently). Second, any non-trivial proposed feature needs to have some existing user experience behind it. For libraries that typically means implementations shipping with operating systems or compilers (but successful third party libraries might also be considered). Finally, it also needs to appeal to people on the committee; that can be quite challenging as well. Many proposals that meet the first two criteria die because they simply don't get enough support within the committee.