So it occurred to me that, just as one can say "der Wagen is rot", one should be able to say
Der Wagen ist mein
with the uninflected (predicative) adjective "mein". That should merely mean, that the car belongs to me.
That is slightly different from
Der Wagen ist meiner
where we are using a possessive pronoun (ie, "der Wagen ist mein Wagen").
If some reader speaks spanish, I interpret the two sentences as meaning
El/Ese coche es mío
El/Ese coche es el mío
The first sentences uses the possessive "mio" as predicative, and it means "der Wagen gehört mir". It just says that the car has the "property" of being mine (like "being red", "being fast", and so on).
The second one uses a pronoun, and it means "among all cars, this is the one that belongs to me". Admittedly not a big difference, but it is definitely there and natives do perceive it.
Does this subtle difference also exist between the two german phrases (assuming the first one is correct)?