The gold cross on the black background would represent St David of Wales's Cross, although when I see the black background I thought of St Piran of Cornwall's Cross.
The problem with that design is that it still includes the white from St Andrew's Cross.
At the moment Wales is not included in the flag. The simple reason is that when Great Britain was formed in 1707 it was the union of the Kingdom of Scotland with the Kingdom of England (which included Wales) - it was not until the Welsh Language Act 1967 that it was made clear that, going forward, references to "England" in legislation did not include Wales.
But can we include Wales now? Yes, with a minor adjustment. So, how about this?: