I have had good experiences applying 4e elite and solo monster type mechanic concepts to regular monsters in 5e (and Pathfinder 1e) for boss types.
For an elite leader of a group fighting with others double their hp, make their single target attacks hit two targets (twin spell or such) or double their attacks, and perhaps give them two reactions or an extra thing to do once off round. Counts as two monsters for xp but lasts and hits harder than a single one while the numbers are still party level appropriate.
For a solo to be a big engaging climax fight give them hp x party member number (or alternately x4 or x5), turn their single target attacks into area ones (blasts, whirlwind attacks, etc.), give them a reaction or action in between every PC's turn, make them immune to or reduce the effects of lockdown effects.
Well, they decided they want to go back and deal with that dragon. They did not long rest in the vault- didn't need to. They were full strength goin in, so only down 1 2nd and 1 6th level slots, and no injuries.
I'm using the 5.5E Monster Manual, and adding the lair actions from the OG 5E. (why did they remove them?? or am I missing this section. likewise, why no bite attack???) Adult Black dragon. This IS it's lair, so I applied the boosts for that already. It's init is around +12 or +13, which is way better than the party's. Throwing those legendary actions in there helps, too! (this time I will remember to use them, dammit!!!)
Making it an Elite monster would definitely toughen it up. I want to do that, too.
I looked up Adult Black Dragon in all 3 Monster manuals: 5E, 5.5E, and Level Up. all 3 were very different monsters!
I liked 5.5e's version best, but it, too is missing something... BITE, tail, wing attacks
Gotta say, I LOVE how they added it's Initiative bonus line to the top of the stat block. Great change! Same for doing the math, and printing it's saves right along with it's ability scores. Another great usability change. I actually started doing this on MY character sheets, new and rewrites, lol.
And finally, rolling spell casting options right into it's actions. Again, a minor change that makes the DM's life a bit easier.
Example: in the multi-attack entry for this dragon, it can make three Rend attacks, or two plus a Melf's Acid Arrow (upcast 1 level)
It also ties the at-willl Fear spell it can cast to it's frightful Presence legendary action. The addition of Pounce as a legendary reaction it CAN repeat, is nice- move and attack once, on someone else's turn.