Now that CQL2 filtering for OGC API is implemented (yay!), it should be a relative small effort to also allow WMS & WFS filtering with Common Query Language, next to filter encoding standard (xml). CQL2 is arguable more concise and readable.

Similar to OGC-API, a vendor specific query parameter filter-lang could be used to specify the filter language (fes, cql2-text or cql2-json). The default for this could be set at map or layer level.