What are the sneakiest things that can be done to Bitcoin via softforks? Or another way to phrase it, which types of softforks are so powerful that they are almost like hardforks?

I figure that the question could spark some interesting replies.

An example is SegWit, which effectively increased the block size without technically increasing it. Another example would be some sort of increasing transaction tax on older UTXOs, where in order to move them one must pay an extra fee determined by how many blocks they have been dormant for.

What else?