Do your tags have structure? Start organizing tags into a tag tree to get some structure. Consider all tags that you currently have as level 1 tags. Then organize all of your 400+ level 1 tags into some 30-100 level 2 tags. Do it bottom up, not top down: that is, do not make a list of level 2 tags and then fill them up, but just start grouping level 1 tags together.
Then when you get a list of 30-100 level 2 tags, then repeat the same thing with level 2 tags - organize them and group them in some 10-20 level 3 tags. Than you should have some kind of high level overview of your interests. You should get a better idea about your areas of interests and about the amount of data you collected for each of the areas.
When you have a clear picture of your data, then make a clear picture of your projects. Use your tag tree to figure out which projects you have in prospect. And then define them clearly: give each of them a title, a summary, a short roadmap.
When you exhausted all of your data, when every tag you have is assigned to a project, then just contemplate the list of your projects for a week, then prioritize them, choose a short-list of 3-4 projects and take another week to work them out in detail.
Then choose one and just begin working on it :-)