Proper compile time sum types would be great. I find myself reimplementing sum types at runtime far too often, especially when it comes to parsing JSON. My go to is a function with a signature along the lines of the reflect packages dynamic select:

func UnmarshalOneOf(data []byte, args []interface{}) (index int, err error)

Which I use like this:

variants := []interface{}{ &T1{}, &T2{}}

i, err := UnmarshalOneOf(data, variants)

// …

return variants[i]