[Ek1]

Nothing new is needed to add as opening the party channel for non-human-approved messages would be enough i.o. sending messages without pressing enter.
As in this has been disabled since beta, there is a slim chance making basic data exchange possible this way. I mean, nothing prevents for a add-on unlocking everything and simply selling or deconstructing all but sending text is the big bad wolf.

If there are ZOS developers free from implementing that AH or developing guilds out of beta phase, a new channel would indeed help me develop dozen or so add-ons that are benched simply because the game lacks a way to exchange data with other players.

If performance hit is the concern, then a channel can be implemented with safe boundaries to ensure things stay in check. One way to do it could be having a buffer channel funktion that accepts messages, queues them and sends them in order after waiting X long since last message sended. Wait time should be server or instance determined cost, different for different channels as in messages that reach zone should have bigger time cost than those going to party or instance. Server sided variable could be adjusted dynamically by server so that when times like present hit the server (ani event) it wont choke, or that each use of channel increases the wait time that reduces only by not using it. Also the add-on needs feedback when it can send again, maybe because of server load or because multiple add-ons use the channel at the moment. Main point is that the add-on can get info how long the wait time is and how it is calculated is so it wont bother server when it wont deliver.

Anycase, I see that there seems to be need for two features or channels.

One for fast and small data exchange.
Be it 8 byte wide and 8 byte deep, so 256 channels with 256 characters with described buffer or a even simpler channel buffer that sends the most latest data it contains to party after waiting server_sided_time_variable milliseconds before sending again. That way channel load can be regulated yet allowing instant data feed if the channel has not been used for at least server_sided_time_variable milliseconds. With int sized channel even exact numbers could be exchanged. I recon that with that amount of channels and data package transfers in a party it taxes the server about as much as using the present mapping data channel. After all, it is using a feature to do stuff that was not designed to do this stuff, yet not widely used.
As a personal note, i doubt even 5% of the channels would be used as ultimate levels is what people want to know. I admit, i haven't been diving much of combat code/data or paid attention to that side of add-on discussions but as vet trial player I been very little interested about other peoples stuff except how much ulti they got (that should be party shared anyway in the first place), how many monsters they are tanking (or better yet, the sum of the chasing enemies HPs), what food buff they got on (simple boolean would work too but more info the merrier), shield strength they got if any and what quest they have active (are we on the same quest).

Another channel would be a way to change longer, less time critical messages.
Simply opening all the channels with the described timed chat buffers works or opening a separate party channel, lets call it party_unrestricted, that accepts messages without user needing to hit enter. That way automated data exchange can be made for something more complex, e.g. like telling my party members that I am missing 67 ashe for my master writ.

That for the feature wish side but what about the can of worms it might open?

My main concern for opening free fast data channel is that as the game is still lacking grown up market place/auction house (not listing every instance of item sold, just one item listed and that items lowest instance price) people start to use the fast channel to stay updated of information that does not move otherwise. As I/O's are closed, people start to figure work around like TTC and MM ,that both also make huge amounts of data pull requests, instead of just simply one per searched item with proper market place/AH implementation to get info about sales.

I know i would be tempted of using such channel to change information of sales with other guild that i am not member. Especially because account is limited to five guilds that can have maximum of 500 players. Ponder the situation where one guild slots goes to RL friends, one or two or three to connect with your nationality/language people, one for getting raids and one for roleplaying/lore/crafting. Well, your out of guild slots to get that MM info so sure i would appreciate i some hardcore trader would share dozen or so trade guilds data with me through a data channel. I can go get tea while that ingame data exchange happens. Hell i might get a separate account to do just trading and multibox the data. All these problems would be solved with reasonable number of allowed guilds, with server wide language channels, with reasonable member numbers for guilds or with that reasonable, adult market implementation.

All of the/these things that are nagging add-on developers and the games metacritic rating are design mistakes, if not outright flaws, since beta. Correcting them would fix lot of problems and also boost sales. The game is a uncut gem.