That would be a fantastic first app!
Looks like would just need to set the parseUrls to false on the message object: https://github.com/RocketChat/Rocket.Chat.Apps-engine/blob/master/src/definition/messages/IMessage.ts#L21
Also if you want something like the forum spoiler… could put in attachment and use the collapsed property to send it collapsed:
https://github.com/RocketChat/Rocket.Chat.Apps-engine/blob/master/src/definition/messages/IMessageAttachment.ts#L12