What are some options on using the blank cards in the The Gathering Storm Expansion?
I haven't got the expansion (only the original game), but I've had a few ideas:


Duplicates. If there are existing cards in the deck which you find
are key and don't have a duplicate, consider making another of those
available. I'm specifically thinking of the cards that score 3VPs on a 6-development, but there's only one copy of it.
Copying but changing colour. If you find there's a really
useful Mining card, but no equivalent for Alien, you could consider
making an Alien version of it.
Looking at existing cards' limits.
e.g. If you're making a 6-development, look at what the maximum
points it's possible to score with the existing ones, and make sure
the ones you make don't exceed that.