Currently we have a few of these:
- ve.dm.MetaList for meta items which currently aren't in the model tree (until T56299: (Re-) unify data and metadata is fixed)
- ve.dm.InternalList
The are used for getting lists of categories and reference contents. Some of the code in these could be de-duplicated and there are other potential uses of a type-indexed cache, such as listing all images in a document for showing in a gallery creation dialog.