Map, which contains biomes, which contain landforms, which contain tiles
I want it to be generic so I can easily add new links to the chain
  (like adding a new kind of section between Biome and Landform for
  example)


Generics work fine for simple generic data structures (like a List). Your "generic" data structure is actually a very special one which will not be used outside of your model. It is more a try to extract the common parts of your model to a generic structure which may be useful, but based on your question I can not see the value in your case.

In my experience, data stuctures with multiple generic types, which has contraints to other generic types, are hard to understand and make the code more complicated.

In your case, I would just give generics up and write the data structure down as it is:

public class Map 
{
    public List&lt;Biome&gt; Biomes { get; } = new List&lt;Biome&gt;();
    public List&lt;Map&gt; AdjacentMaps { get; } = new List&lt;Map&gt;();
}

public class Biome 
{
    public Map Map {get; }
    public List&lt;Landform&gt; Landforms { get; } = new List&lt;Landform&gt;();
    public List&lt;Biome&gt; AdjacentBiomes { get; } = new List&lt;Biome&gt;();
}

public class Landform
{
    public Biome Biome {get; }
    public List&lt;Tile&gt; Tiles { get; } = new List&lt;Tile&gt;();
    public List&lt;Landform&gt; AdjacentLandforms { get; } = new List&lt;Landform&gt;();
}

public class Tile
{
    public Landform Landform {get; }
    public List&lt;Tile&gt; AdjacentTiles { get; } = new List&lt;Tile&gt;();
}


Much more readable! The properties has more descriptive names and it needs a minute to extend this hierachical data structure with other types.