The next time you land at Changi, have a good look out of the taxi window as you set off down the East Coast Parkway into town. A good long straight stretch, central reservation easily removable flower boxes, lamp posts not solid but screwed down to the ground, a few easily moved trees=an emergency runway.
There seems to be a similar straight stretch of road just to the west of Tengah and a possible but short stretch by Paya Lebar. Mind you, may not be needed with the F35B on order.