Pack light, pack only essentials, wrap clothes/gadgets etc in durable-reusable plastic cover though you have rain cover for your backpack, be respectful of local traditions and ask permission before clicking pictures of locals (some do not like it)... Have a awesome trip buddy...
Unlike the North or the South, North East is still barely explored. Make sure you carry spares ( clutch & breaks cables, puncture kit, inflators ) as finding repair shops beyond Assam will be rare. Food too is not easily found on the highway beyond Assam. Locals are very vary of visitors and strangers, make sure you follow traffic rules and respect the local culture. Avoid riding at night or after Sun down. I started my journey of thump riding in the north east and the terrain is beautiful. Ride on!
December will be cold. But not freezing, unless you plan to go all the way to tawang. Arunachal gets some snow fall post 20th December.. Early December is perfect, especially through Assam, Meghalaya, Nagaland