Imho a big problem in "fixing" marksmen is that they're all lumped together in the same subclass
Even though they play more similar to other subclasses than to one another: Sivir plays a lot like a battlemage Ezreal plays a lot like an artillery mage Vayne plays a lot like a skirmisher Etc
Imho the first step in actually making them viable and balanced is to finally recognize this, to finally recognize that, for example, when discussing balance Sivir is better associated with Karthus than with Jihn.