These are all good ideas. Also, it's possible to prolong battles by using separate waves of monsters, thus giving some of the lengthier abilities time to matter before the fight is over, but doing this repeatedly would start to feel contrived. (Along the lines of Dragon Age II turning every single fight, no matter how minor, into a battle with a huge number of enemies who seem to endlessly parachute in out of nowhere even when there couldn't possibly be that many people involved in the situation.)Since 5e monsters have shorter lifespans, constant or reaction effects are better than action abilities.
Persistent zones from 4e are pretty good if you want a constant effect throughout the battle, even simple zones like 'All ground is considered rough terrain in a 15ft zone around this creature.' I imagine this would work for some kind of quaking monster: a giant, or earth elemental.
Reaction abilities like fear effects, healing, or other damaging effects are also fun ways to make a monster more interesting. Like: as a reaction an undead-something-or-other that heals anytime a necromancy spell is cast within X feet of it. Imagining that the undead siphons some of the energy out of the spell into itself.