So, north underrail is going to be highly anticipated by previous fans but I've seen quite a few people hoping that the player character won't have to start from scratch. There's a couple of ways you could deal with this.
Personally, I would like to see a 'Mass Effect' system where it analyses your save game from Underrail and carries across important choices. For example, if you sided with the free drones then the northern underrail free-drones know about you and invite you to join them without having to do pre-requisite things. The protectorate will likewise start with a negative disposition toward you, though perhaps not fully hostile, since I imagine their presence will be much more pronounced in north underrail.
However I wouldn't like to see characters being copied across, or at least an option for your choices to be carried without your character also being carried. Building a character up through the game is a lot of fun and I wouldn't want to miss out on that just to keep my old guy. Plus, I'm not sure how my old build would do in a new world.
Another option is to carry across characters and equipped gear from the first game and have the game balanced so that your old stuff seems really weak compared to new gear. That and the level cap gets doubled. I think this would be a more difficult option from a game balancing point of view. I suppose you could always balance the game for new starts and allow people to export and import their characters like in Underrail.
A third option could be something like your train crashes at the start of the game. Your character gets amnesia and you basically have to start again but later in the game you can hunt down groups of scavengers that stole your old equipment and pay a memory expert to recover your lost abilities/psi powers. That way you could get your awesome stuff from the first game back without it making you overpowered for the start of the game.