Deny me-sign over a low-hp allied hero when deniable
Well, this might seem a bit unnecessary, but let me tell why I would like an addon like this.
When playing pubs, most newbies aren't aware of the fact that they can deny an allied hero who has a DoT on him. I don't know how many times I've runned to an allied hero with Slither's dots ticking on me while desperately trying to write "deny me!" in the chat (often coming out something like "denyi!" "drney1!" in all my haste), and then I've died next to them while they're standing there stupidly starting at my corpse.
If we had an addon that displayed a red "Deny me!" sign over heroes health bar when they go below a certain % of HP (15, maybe?) and when they have an active DoT on them, our fellow newbies might get the hint and actually try to deny you, thus denying the other team experience and gold. I think this could come out rather handy, saving you a lot of trouble since you wouldn't have to try to spit out the word "deny me!" while trying to position yourself next to a moving allied hero.
I don't know about the coding and how the addon could actually figure out when you have a DoT on you and when not, but I'm sure you magicians could figure something out.