I’d say it depends on age, partly because you can give add-ons as gifts for several years!
A tunnel and a bridge sound perfect to start. I’d look for a solid bridge that’s easy for little hands to manage rather than one that balances on blocks or a suspension type that has sides in the way of little hands.
I’d hold off on junctions and crossings for a while because it can be quite frustrating switching between male/female track sections (I think it might be the ikea track that has plastic connectors that can be switched out with brute force )
I’d hold out on battery operated trains for a good while as I think they change the way they play. Also the connectors can be cuddly and frustrating until the fine motor control catches up.
As they get more creative with track you can add more pieces. Bendy track is great for finishing awkward designs.
If he likes Thomas & Friends there are some nice pieces like the sheds where all the trains sleep. Some are better for slightly older ones; We got Cranky the Crane at 6 and he needs to be gently operated.