Existing MPX modules are: Radeon Pro 580X, Radeon Pro Vega II, Radeon Pro Vega II Duo, Radeon Pro W5500X, Radeon Pro W5700X, Radeon Pro W6600X, Radeon Pro W6800X, Radeon Pro W6800X Duo, Radeon Pro W6900X.You are correct, it would be the W7000 series GPU that Apple would use, not the RX series.
There is no Radeon Pro W6700 in AMD's line-up, and no driver support for RX6700/W6700 in macOS.
If Apple ever uses Navi 3 (that's a big 'if'), it seems fair to expect a "Radeon Pro W7900X" and/or "Radeon Pro W7800X (Duo)".