I have mixed feelings about a "5th slam". If there were one, I believe South America would be the better choice. But I think it would be difficult to fit one in, and I lean to the side that the traditional 4 shouldn't be changed.
I'd rather see a masters on Grass. It's long, long, long, overdue. It's rather silly for me to see 3 masters on clay, 6 on hard courts and nothing on grass.
If it were up to me, I would have 4 masters on clay, with one in South America. I would have 5 hard court masters, and at least 1 grass masters if not two, 1 in Europe, and 1 in the USA.
Respectfully,
masterclass