How to plan a north-east trip?

1. Ideal length should be atleast 8-10 days.

2. Will suggest some of my favorite offbeat places Arunachal - Tawang axis, Mechuka-Ziro axis, Anini-Dong axis
   Assam - Manas NP, Haflong, don't miss Shibsagar if you're a history buff Meghalaya - Garo hills, Balpakram NP Sikkim - Lachen, Lachung Nagaland - Would love to suggest Ukhrul if you can find good contacts

3. Best time to visit is September-October. March-Sept is peak rainy season.

4. I personally prefer and suggest renting an Innova for your entire journey. Why? - You don't want to feel exhausted at the end of the day riding daily, roads are getting better but still not the best. Innova is a comfortable ride on long roads and on bumpier sections. You can self drive if you are really skilled but you should know to drive through deep mud and broken roads, and icy sections in parts of arunachal.

5. Rough maths. Regular homestays (most prefered options): 2-5k per night including breakfast and dinner, Innova: 5-6k per day including driver and fuel costs. Smaller vehicles cost between 2-3k per day all inclusive. Daily meals: Can be anywhere between 500-1000 bucks depending on if you're having meals at homestay or away

6. People are extremely friendly and family friendly. Embrace the local culture, go with an open mind, bamboo shoots and fermented fish are common ingredients in local cuisine - let your hosts know if the smell is strong for you, try local fermented liquor and rice wines. Otherwise you get regular mainland Indian food at most homestays. Please don't litter and carry trashes, don't wander late at night, and avoid touching local folk shrines. Don't stare at the women too hard - you will be mesmerized. Trekking options are limited compared to HP, Uttarakhand, Ladakh but things can be arranged through your local hosts. You will hear anti India talks in Nagaland openly - just nod and say yes, don't engage in political talks there

Good luck, and wish you have a great trip there.

Dont try to cover all 7 sisters at once. Start with Assam and Meghalaya first. In around end of the year, you can visit Hornbill festival in Nagaland.

Best time visit would be in Winters. Try Meghalaya around Christmas

Where do you specifically want to visit? 7 sisters visiting in one go can be unreasonable. Some of the places will be a waste of your time. Be specific about your destinations then only I can help. Some recommended places would be: Meghalaya Kashi Hills region, then there is a Sikkim trip that includes Bhutan as well. In Assam, you can visit Kaziranga, New Half long and there are 1-2 places in Arunachal Pradesh as well like Taiwang.

Its difficult to cover whole 7 states on one go, unless youre planning workation (or a long holiday). Either you'll miss out some places or you'll be exhausted ( dont streatch a trip long until you're bored).
We did bike trip covering Assam and Meghalaya in 12 days. You can do it in 8 days if you hire a taxi. Meghalaya has beautiful waterfalls and Dawki is a good place to get photos.