EDIT: I know how you say that writing => other ideas. So I guess the question is whether
1) You have something else in mind currently
2) You're writing with the intent to come up with other ideas
3) You're writing, and if another idea happens to come up that you like, you'll pursue it, or
4) You want to write and not pursue other ideas
I have a whole bunch of ideas for essays I'd like to write, but I almost never pick a topic. The way I decide which one to write is that one starts to write itself in my head.
As someone who only comments in two places on the web (HN and at another place that I'd rather not mention) that is exactly how it works when I make comments.
You read something you react you write.
It's not an assignment. It's not a job. It's not required. You just do it because you are moved and/or inspired by something you read or observe.
Just a thought (I know you've got a huge "to-do list" in your head), I'd like to hear your opinions on effective altruism, and what you think the best ways to change the world are.