Add info to mob tool tip like dangerous casts. CC Immunity and stuff.
I think this will greatly increase the user base since imo there will be alot of users that dont pre plan pulls (or use preset pulls if that is planned) but need info about specific mobs to focus/care about/how to handle them.
I dont know where you wanna go with that addon. I think that is alot of work and goes more toward some kind of m+ compendium.

• add npcID and/or link to wowhead for getting more info about mobs.

I love this suggestion.

The tricky part here would be to present all of this information and still keep the UI clean. Perhaps just simply a clickable Wowhead link for each mob? That you can click on and investigate outside of the addon?

It looks like Wowhead uses the URL format: http://www.wowhead.com/npc=XXXX#abilities where XXXX is the NPC ID.

This has been implemented into the newest version. Data is currently being gathered by the userbase.