Personally, I don't feel like the existing 4 tanks are different enough to warrant a 5th. If there aren't enough ideas to spread across 4 tanks, then there won't be enough for 5.

And to be clear, I'm not even talking about whether they're the most effective at X, Y, or Z; it's just whether they feel different, both thematically and functionally. And in my subjective opinion, they don't; they're 90% the same moves with a tiny dash of flavor.

That said, when 8.0 arrives, we will 100% be getting a new tank; the only question will be what the state of the existing 4 tanks will be at that point, and whether the 5th can bring any new ideas to the table.