The Cromwell would be a fine addition to AH, no doubt. If offers a fast moving tank (some sources say 40mph), but with average at best AP ability. Its HE capability would be the same as the M4A3 Sherman 75mm (same ammo, different gun). So... until other gaps are filled the Cromwell might be on the back burner.
The British Valentine tank, with both 2 Pdr and 6 Pdr guns optional would be a great EW/MW fit for AH, imo.