Maybe even Australia too??
Or just spend more time in Indonesia...there's many islands.
How long do you have? That will help us suggest some places.
Have a look at sky scanner. That might help your decision.
Just a thought however, that time of year is peak season for tourism so you would be wise to make a quick decision and then book, both flights and accommodation.
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December weather is pretty good in Thailand, Cambodia, Laos and Malaysia, so those might be good choices if you get rained on too much on Indonesia.
December is high season all the way around Asia especially Christmas week and new year so book flights and hotels asap as they will already be expensive and options getting more limited.