What about the vowels in words such as say and know? How might they be pronounced in a standard accent that can be promoted by teachers? (Here, I will follow the suggestions of John Wells and refer to them as FACE and GOAT. This way, we avoid prescriptive statements about which pronunciation is "correct".)
These two vowels vary quite considerably in Englishes around the world:
- in England, they tend to be pronounced as diphthongs: [eɪ] and [əʊ]
- in Australia and New Zealand, the starting point is rather more open, and they might be transcribed as [æe] and [ao]
- in the USA, they are diphthongs for some speakers, but especially for GOAT, there is less change in quality than in England, so this vowel is generally shown as [oʊ]
- in Scotland and Wales, they tend to be monophthongs that can be transcribed as [e:] and [o:] (though length is not generally shown for Scottish English)
- in many parts of the world, including India, East Africa, West Africa, the Philippines, Malaysia, and Singapore, these vowels are monophthongs that might be shown as [e:] and [o:]
If speakers are going to England, Australia or New Zealand, maybe a diphthong is best. But for the rest of the world, a monophthong seems to be the most common pronunciation, and this probably achieves the highest intelligibility in a world setting.
This, then, is another feature of pronunciation where the most common pronunciation found in Singapore can be encouraged, as it is internationally intelligible. Furthermore, using a diphthongal pronunciation for these two vowels makes the speaker sound awfully British, which is something most Singaporeans probably want to avoid!