I suspect that the future fleet of BA, like many airlines, would be an A320/321 NEO mix or 737-8/-9 MAX. Don't think the smaller A319/737-7 will be too popular with the heavier engines and wings, much like the -500 was a popular classic, but the -600 was considered something of a dog. The consensus in the industry seems to be the A320 and the 737-8 are the baseline models.