What is a suitable indefinite pronoun that fits into the phrase?: "All pens are bad -------- seem to be writing."
There are two indefinite pronouns which are appropriate for this sentence, but choosing the correct one depends on the meaning of the phrase. If there is a context, you can use it to find clues about what the author meant; if there is not a context, I would say that it is based on your own opinion.
The first potential indefinite pronoun is "all," so the phrase would be: All pens are bad; all seem to be writing. (The semicolon is necessary to make the sentence grammatically correct, though you could also use an em dash.) The meaning behind this one is pretty obvious: pens are bad because they write (presumably on their own, with no human help), which would be a little disconcerting to see.
The second potential indefinite pronoun is "none," so the phrase would be: All pens are bad; none seem to be writing. This version of the saying is more humorous, saying that pens are bad because they do not write on their own.
Either pronoun is correct grammatically, but the correct meaning is based on context clues and/or your own choice.