Several translatewiki projects use $1, $2, $3, etc. as a parameter format. They should all use the same Insertable class, and it should be in the Translate extension rather than in the translatewiki repository.
Some projects, like MediaWiki and its extensions, use $1 and also {{GENDER}}, {{PLURAL}}, $letters (for tvar), etc.
If it's possible to put $1 in one class and the other insertables in other classes and then use them all, it would be nice, but not a requirement for this task. It's enough to get all the projects that need only $1 to use the same class.
@Aklapper, this can be done in Google-Code-In-2016.