Adding an explicit pre-processor would be great. They could turn commands such as {{today}} into the correct link (e.g. when you open a daily note, the title could be {{today}}, today would be rendered as 2020-06-08). Same could work for {{yesterday}} and {{tomorrow}}, {{this week}} and so on. Of course, having a general pre-processor goes beyond mere relative links, they could also be used for loading remote resources, or even running general scripts!