As somebody who has struggled to bring ideas from concept to completion the best advice I can give is keep writing drafts, but do not force an idea down your own throat.
The only way to master Foundation tone is through reading scps, writing drafts, and getting those drafts reviewed. On the other hand, you will have more success when you are writing about an idea that genuinely interests/scares/amuses you and when you have a story you genuinely want to tell rather than when you try to write an article about an object that doesn't hold even your own interest.
Furthermore, and this is only my opinion, do not worry yourself about the ideas themselves. The difference between 'good' and 'bad' ideas is exceedingly slim: execution can save the worst or ruin the best. Some topics (humanoids, meters, memetics, compulsions) carry special challenges- if you don't have experience with the site, you won't know what mistakes to avoid when writing about these sensitive themes. Beyond that, however, the article will be good if you write it well and communicate a story to the audience.
Ideas on their own are so minimal they can very rarely be judged. Often people will say "we already have that" in response to the bare bones of an idea, but in execution similar seeds produce totally different results. So don't get hung up on finding the perfect SCP to write about, make a SCP perfect by writing about it!