Text alignment looks like it should be easy enough to add without adding too much to the base size! I've created a request ticket on the project so I can keep it in mind when I'm working on the next update!

As for GIFs, that should already be doable in exactly the same way as adding a regular image! Just get the image URL where the .gif file lives and add it as an external image. Unfortunately adding the ability to have an internal GIF image would be a bit too much for Feather Wiki to handle since they tend to be so huge. What you can do, however, is put the .gif file in the same folder as your wiki's HTML file and reference "./mygif.gif" as an external image (where "./" refers to "the same folder as this HTML file").

And video embeds are all so wildly different that it'd frankly be impossible to handle them all—those you'll unfortunately need to just add using HTML by yourself... Sorry about that!