Congrats Scorpious on your almost 100!
About specials: I think it depends of engagement and the kind of attention you have what kind of special you share.
Here is a list of ideas:
1-A cool picture of the MC or the main cast to celebrate the milestone. (is kinda common in Tapas, but not universal) Perhaps a GIF if you know how to do them?
2- A Q/A sesion. They can learn more about the characters and the world. I recommend you to make sure you get enough engagement and readers who comment.
3-A short character popularity poll. Readers can share their opinions AND you can learn some valuable info. I also recommend to do this when you notice enough engagement.
4-Sharing the guest art and fanart people sent you, crediting them. This is cool to show your appreciation to your readers and maybe meet some artists in the proccess. A tip to get some guest art is going to the topics of "i will draw your OC". And about getting fanart.... Is something that happens spontaneously.
5-If you have the extra time you can do a "special week/day" when you upload more than usual. Maybe a double update or one update every day of the week if you are feeling wild.
Which ones work for you better will depends of your situation.