I would try to match creatures to the dragon that are immune to it's type of breathe weapon. Then the Dragon is free to use that regardless of where it's minions are. Not that the Dragons care about the minions as much as it's tiresome overseeing the training of new guards.
The way I see it is to have the characters plane-hopping, and interacting with the "icons", but on a quest that is vitally important, but doesn't lead to the destruction of the different settings. It may be that something unique is needed from each official setting to destroy an evil artefact...
In our group I run a annual "Christmas Bash". Every year is a different theme. It is generally a last creature standing. Sometimes I pregenerate characters for them, sometimes each player gets a monster to play, usually they have to create a one off character within strict boundary's. It...
Figure out where the rogue is likely to hide and have a hostile creature or two already hiding there.
Or have that area trapped somehow. Even if the rogue spots the trap dealing with it is likely to attract attention.
I like that the designers are getting the views of the players through the UA articles. Some things will make it into releases like SCAG. Other ideas will be dropped.
I doubt any idea will have universal acclaim.
Strangely enough one of my players, also running a bard, was aged by a ghost. Further into the dungeon they encountered bearded devils. The player bartered his characters soul for returning the characters normal age.
I allow any feat from the PHB. I do not have time to create new feats. However, if a player proposed a feat then I would scrutinise it, maybe suggest modifications, then if I was happy with it would allow it to be play tested. As DM I would reserve the right too stop the use of the feat, or...
I have just finished running my group through "Tears for Twilight Hollow" from Dungeon 90. It's co-written by Angel Leigh McCoy & Christopher Perkins. Very good scenario for four by 7th but easily adaptable
The only the reason I can think of for a wizard to use weapons is in a "magic suppression zone" . Or using the dagger to carve of a slice of meat at the dinner table.