Not merger. Working together as a bigger economy scale like Europe. Same currency. Reduce cost and time. Also can reduce defence spending no need to compete with each others.
Same as US even different state have different law and gov. If they don't like it they can seperate out as Independance state/country. Only different is they have been working together for last century. Where as Europe only working together for one decade.
For begining maybe can start same as Europe.
Brunei, Malaysia, Singapore and Thailand join together for one currency. Slowly will ad in Veitnam and Indonesia. If all join we will have bigger population than US almost half of China population. Imagine economy scale of this. Maybe then new Asean currency will be one of the major and important currency after US$ and EURO$.