I personally think it’s an awesome collection goal to have your favorite artwork of every Pokémon in the National Index including their forms.
I personally just finished the complete National index. Was re-organizing my collection after buying a lot with 7k Japanese cards about a year ago. It turned out I had most of the Pokémon already. Was missing like 50 regular ones (including forms) and most of the Mega Pokémon.
In case you are going for it including every form, I’ve just posted a list of all Pokémons with multiple forms as well as which are and aren’t available as TCG cards.
Greetz,
Quuador