At a first guess, there are three main pieces to this work:

1. Adding logic to the FieldPath class to notice and accept something like "$[0]".
2. Adding logic to the Document and Value classes to interpret this correctly in methods like 'getDottedPath'
3. Adding logic to the $project and $addFields stages to support something like {$addFields: {"a.$[0]": 1}} - $project might be slightly weirder with its semantics of how anything not referenced is not included.