4 month backpacking trip (October 2026 - Januray 2027): need some help with my itinerary.

Hi Folks, I am planning to do a 4 month (17 week) backpacking trip through SEA from october 2026 to january 2027. While I am planning to go with the flow while I'm there, I'm also creating a detailed itinerary to fall back on and to make sure I can book thing in advance if necessary. Right now I have an itinerary for 3 months, and I'd like some suggestions on how to spend the extra month worth of time.

My itinerary so far:

In short:

• Slightly over 2 weeks in northern Thailand
• 1 in week Laos
• Between 3-4 in weeks Vietnam
• 1 week in Cambodia
• Between 2-3 weeks in Southern Thailand & the islands,
• Almost 2 weeks in Malaysia/Singapore

In full:

So I still have the whole of Januray left. The options I've thought of to spend this extra time are:

1. Spend the extra month in Indonesia. In this case it's probably best to follow the route in the opposite direction due to January being the rainiest month there. On thing I like about Indonesia is the possibility to learn diving and/or surfing there (never done either).
2. Spend half a month on slowing down my existing plan and half a month in Borneo or Indonesia. In this case it might also be best to reverse my current route.
3. Spend the whole month of time to slow down my current itinerar and maybe add some destinations in between.

Which option do you suggest? And which locations would you add/ spend more time in?