...almost looks wide enough for a whole island with two extra tracks there.
I disagree and why well for starters if you look at the width of the existing island platforms there then look at the space that's available and include space for the infrastructure such as platform and OHL etc, you're only going to have enough space there for a single platform especially as it is meant to have a lift as well.
If they were to put a island platform there then it would be a narrow one which could well present issues in the future.
Saying that if Network Rail brought some of the car park beside the proposed site then yes your island platform could happen but why would you need a island platform when a single face platform with passive support to join the Down Slow in the future is all you really need.
Least when Platform 5 is built, you could in theory in the peaks extend more of the Hertford terminators thought to Stevenage so upgrading from a half hourly service to a 15 minute frequency, that is of course all theory.