I mean spellcheck might actually make a lot of sense (if the hooks were appropriate). The interface is mostly set, but the word list and how you figure out if words in the document are correct have a lot of potential variation. Selling the product seems like a lot of work though. Maybe if you could add spell check in a language not included in the box, that would have at least a chance regionally.

But things that are maybe even more potentially useful like math formula editors get really hairy if you want to share documents. Then people viewing the customer's documents might need a special viewer or a licensed copy of the editor if they want to adjust things. It's a big mess.