A GM can never have too many resources at their fingertips when running a game. That's my honest opinion. Creating everything from scratch is hard, so having pre-made monsters of all type, in particular, is very helpful. That's where the people of Carwood Publishing come in with thier Monsters of the City series of books. There's a new one up on Kickstarter called Sins & Virtues that has 100 more pre-made monsters for you to throw at your party.
From the campaign:
Monsters of the City is the third book in our 5E Monster Series. With the help of hundreds of backers we've already published Monsters of Feyland and Monsters of the Underworld. Now we enter the urban environment to find an epic battle between good and evil. Another member of the Alliance of the Griffon, a truth teller named Greta Goldheart, has just arrived to bolster the forces of good.
7 sins, 7 virtues, and 7 districts in the city! Now it's time to choose a side! This book will contain 100 new 5E monsters (approximately 130 pages). A district page with additional lore and tables will begin each section. There will also be encounter tables, adventure hooks, GM advice, events tables, locations tables, CR and creature type listings, and more.
Each district will contain two legendary monsters that represent one of the sins and one of the virtues (see below). Monsters in the book include devils, demons, celestials, fey, undead, constructs, oozes, giants, and many humanoids. In fact, three new races of humanoids will be introduced. The book is softcover to begin with, and there is a stretch goal for a hardcover version.
The campaign's a little more than halfway to its goal with 20 days left to go.