The stream viewing options on desktop include "fullscreen" (no chat displayed) and "theater mode" (chat occupies about 1/3rd of the screen, on the right). Given that the vast majority of streams are in widescreen format, dedicating a large vertical column to chat results in the gameplay "window" occupying less than 1/2 the screen, with huge "letterboxing".
It would be a huge improvement to allow the viewer's chat display to overlap onto the video feed, via a window that can resized, repositioned and whose background can be made transparent. It would serve as an "interactive fullscreen" mode, where viewers can be immersed in the gaming (since it fills the screen) and also follow chat to whatever extent they desire (some may shrink the window and have just the last two entries overlaid in the top-corner of the stream feed, others can have a long narrow column of chat, etc.).

I find pop-out chat to be a poor substitute for this type of functionality ~ it's easy to dismiss accidentally, has large needless borders and cannot be made transparent.

Scrolling through all of the available emotes (especially on mobile) is a hassle. The frequently used emote section isn't that helpful towards this issue either.
I'd rather be able to customize an emote bar full of my favorite emotes, or even have multiple customized emote sections to sort them out quickly and efficiently. Perhaps consider allowing users to name the different custom emote sections. I would actually prefer removing (or having an option to remove) the frequently used emote section completely.

I think just making the emote/sub emote layout a bit more customizable overall would be well-received. It would be more personalized and user-friendly.

Chat latency needs to be improved upon, because it's constantly trying to send and receive 0 Bytes of data which in turn causes the chat to need to constantly "reconnect". I've monitored over the period of 20 minutes to an hour where the chat is constantly flooded with "Welcome to chat", and the network monitor shows the Twitch chat API constantly sending 0 Bytes taking up to 1.0 minutes to 1.6 minutes, where else everything takes ⅓ of that to load at at rate of around 3 ms - 400ms.

wss://irc-ws.chat.twitch.tv/ Websocket takes up 1 minute to 1.6 minutes to receive any data when it's not receiving anything at all. Second to this culprit is wss://pubsub-edge.twitch.tv/v1 which also sends and receives 0 bytes of data and taking up to 1 minute to 1.6 minutes to respond. They both send error response nonce after doing PING PONG response while listening to an open socket. The connection time takes 1.8 seconds, but it responds in 1 minute to 1.6 minutes, this is too long of a response time to check if the connection is still alive.

Currently the sets of emotes in the menu are locked into a specific order, which means you'll often need to waste time scrolling past emotes you'll never use in order to reach the ones you're looking for. The lists also aren't consistent across platforms; for example, my subscription emotes are directly below my recents on the desktop site, but are sandwiched between two sets of permanently unlocked emotes in the iOS and Android apps. (The ability to manually remove these permanent emotes from your account would also be nice.)

With the introduction of channel points and the ability to exchange them for individual emotes, plus gifted emotes such as this month's RPG MegaCheer event, people are going to have even more separate sets in their menus. Therefore the ability for the user to organise them (on all platforms) will be essential moving forward; rearranging emotes within a set is perhaps excessive, but reordering the sets could be as simple as a drag-and-drop list like the auto-host list.

The way I envision it, there would be boxes for
- Global emotes
- Each set of unlockables
- Each channel you currently have an emote from
- The position in the list where new sets should appear
with the order of the boxes dictating the order in which each set of emotes will appear in your menu. An option to hide individual sets from the emote menu entirely would also be useful.

It could also be implemented in a way that only has three boxes - Global, Channels and Unlocks - which would then allow for automatic ordering within the latter two categories. These options could include alphabetical order and order obtained (oldest or newest first), and perhaps even grouping by method obtained (subsciptions, gift subs, and channel points).