entity_id completion is not something that can be done with AD.
just because you can edit the files from AD with ANY kind of texteditor that works with plain text. (wordpad, word, etc.) so in that case you need another program that is aware from the entities. (might be something that the addon can do for hassio)
RTSP streaming in a browser is extremely complicated. i am not able to find any kind of possibility without using some external application. i have been trying myself, but if i can find a way to play RTSP with a browser its impossible to integrate it in AD.
browsers dont support RTSP on there own.
for climate there are som many people wishing so many things and there are hundreds of climate widgets online available that can be used in an iframe. its hard to create something like that, for sure when you dont use it (like me)
same goes for mediaplayer. i really dont see the use for mediaplayers on a dashboard.
you might want to start a radiostation on a media device (use a switch) or might want to have the option to chose from several stations (use an input_select) you might want to type the name of a song or artist (use input_text) you might want to control volume (use input_slider)
i have a start and stop switch on the dashboard for radio. and everything else goes through alexa
i must say that i havent seen much people experimenting with mediaplayers in dashboard. if so we would probably already have an example from something better