I fear there's no such thing like an Age to do list.
Age writing, when you do it alone, isn't like doing step by step and putting a check mark behind a task description after you've finished part of it. It is more like a cyclic workflow, you continuously walk through the same actions again. Imagine a flowchart with loops in it.
If you have a team of 10 or 15 developers working in parallel on an Age, the workflow would be a bit different; there would be a phase first where all collect ideas, then conceptual design, define if there should be puzzles, draw some concepts and write a backstory, then define how large the Age is together with creating a map, define how the puzzles should work, then part of the devs start with creating models, while others do script programming for puzzles and other machinery, and another person is doing the sounds and music. And there may be a person who is going outside to take lots of textures with his camera, or create them from scratch in a paint program - in case the team does not want to buy them elsewhere.
But we are usually alone, so it's better to cycle through the above mentioned list over and over again, and work on a part we're in a mood for. We don't get a feedback from others in our team, so doing a ready-to-use conceptual design in the first place is very hard. It works better if you let your Age grow while you are working on it. This gives you a feeling of success for each model you've build, and it keeps you interested.
To avoid giving up when you're stuck, understand Age writing as it is: It's writing. Working in Blender or Max is only the realization part. But basically, creating an Age is based on writing it.
So, if you're stuck, go write a backstory, or expand it. That story does not need necessarily be fully put in a book in your Age; you do this for yourself. It will give you ideas.
Or write short stories like journals, desribing what has happend in the Age or to the visitor some days ago. Like "we had a severe thunder and lightning event last week". This would imply for example a tree had been hit by a lightning flash, which in turn gives you the idea that you could add a broken tree somewhere. Then go build that tree, and if you feel you should now, texture it.
You could also make drawings or paintings of items you could imagine in your Age, something that is related to your story. Again, this has not to be added to a book, it's only for yourself primarily. So it does not matter if you're good in drawing or painting, just do it. If you're good, however, you have nice material to add in a book.
You've started somehow in the middle, by creating a map first. You're missing the story. So go cycle back to that part. And always cycle back, let it grow and expand.
It you're still stuck, go take some textures in nature or your town - this wil give you ideas as well. Or browse the web for photos, no matter what. Not textures, search for images of nice places in the world.
If still stuck, don't give up - just make a pause. Age writing isn't a job, it's a passion. Go back to your project when you're inspired again.
Take your time. I hope I'm not frightening you with this, but be prepared writing an Age will take many months, and it can take years.