Terai / Ganges plain

"Anyone out here with some basic knowledge about the Terai (Nepal) or the Ganges plain (India)?"

The Gangetic Plain is basically flatland India, and packed to the nines with people. scratch it off your list. The Terai is the heavily forested semi tropical lowland part of Nepal. I don't know a lot about it other than that you will need to do some serious research if you intend to hike there.

"I'm about to hike there, but without much preparation. What material should I take, and what precautions should I take? (wild animals, swamps, bandits, …)"

You would be foolhardy to hike there without much preparation.

"If you also happen to know about trails or favorable regions, that's a great bonus!"

Your best bet for hiking in India would be in Himachal State or Ladakh, where trekking is a well established activity, IMO. It is also very beautiful up in Sikkim, but there are a lot of restrictions due to its proximity to China. In Nepal, there are many beautiful areas to trek in up in the Himalayas. The Khumbu, Rolwaling, Makalu regions come t mind, but there are many others of varying remoteness as well. No matter where you decide to hike over there, it is absolutely critical that you prepare in advance. Trying to do it without much preparation will not end well for you.

I'm reminded of the throwaway line from my then Swami above Rishikesh, when I was sent to walk down the access road and close the ashram gates for the night. "You had better carry a long spear; the leopards around here fancy our dogs for a snack". (He had two dogs in the ashram).

However, Zanskar I can confirm is great trekking; you get to meet wild yaks face-to-face on lonely passes; and once I had the strong sensation I was being stalked by a wolf for several hours as I hiked solo down a track.