Try adding hordes of mobs and times where the boss has a weak point at certain times. Add different moves for the boss and try to alternating the different moves.
For creating the boss itself, try using armor stands or some mods combined with commands as redstone wont be too useful but still use it.
For boss ideas itself, it depends on the theme.
For a medieval/fantasy setting it could be a dragon, sea creature, or giants.
For cyberpunk/futuristic, a robot of some sorts or just some half cyborg type guy like Doc Oc from spiderman, or iron man from the avengers.
And For a fallout/wasteland theme, maybe try a mutant creature.
Also try using mods to help you and yt guides.
Hope this helped!
