In terms of product, I can see so many ways this could go: everything from here on is pure spitballing speculation and not based on any inside knowledge.

Adding a row of first class suites: not likely IMHO, would make more sense to replace and upgrade business class.

New business class: I reckon this is the most likely outcome, and would either mean another new product to the mix or an A350 debut for the 777X business class suite, which I believe is about ready to roll at this stage. (The thing is, was the 777X business class so purely designed for the 777X that it would rely on making full use of the 777X's slightly greater cabin width of 5.96m compared to the narrower 5.61m of the A350? Or was it also designed with the A350 in mind, so the aisle width is sufficient on the A350 and a bit more so on the 777X)

Keeping premium economy: that'd be a totally commercially-driven decision (well, aren't they all!) based on current and especially predicted demand both in premium economy and business.

Adding economy class: again, would there be sufficient demand and would that offset the reduced revenue from premium cabins? I'm thinking this can be ruled out.

An all-business class config: now that'd be the wild card, to make it all business class with new doored suites. And again, if SQ sees the demand is there to support this (delivering higher revenue overall than splitting the A350 with two cabins) I could see this happening.