A while ago I discussed the rationale for ASEAN 10+n.
I have there explained why it is more practical and realistic to add one national each time. The reasoning is quite straightforward: imagine getting 20 people to agree on which restaurant to dine, this is why APEC never accomplished any thing, at all!
Now there are political issues, in that Japan and US is worried that China may be able to influence ASEAN by getting in first. That may be true, but eventually it would be 10+6, and even larger, to China's own interests. So even if it would be 10+1 for a few years, that is only a temporary set up.
From ASEAN's perspective, since it is more practical to add one nation at a time, the question is basically "which nation"? Ideally one would like to add the largest economic body first, because they will be negotiating together to the next new member. Therefore, the US would be the No 1 choice for ASEAN. I can bet that if US comes to ASEAN today and said they would like to establish a 10+1 FTA, with 1=USA, ASEAN would go right ahead, passing China.
Similarly for Japan, if it were 10 years ago. ASEAN would take Japan as the second priority only after US.
The question now for ASEAN, since as we all know US does not want to be "the one", is whether Japan or China will be the "one". So far China has been enthusiastic and Japan has not shown any interest in wanting to be the "one", the best path toward a larger FTA for ASEAN is adding 1, 3, 6 in sequence.
To conclude, there is no other way to an FTA that the current path of 10+1+2+3. Since it is the most practical and reasonable, it is inevitable. Nothing can stop this unless China (or ASEAN) changes its mind. That is very unlikely, and unwise for China or ASEAN .