Assalaamu alaykum. {Have you not seen that Allah created the heavens and the earth in truth? If He wills, He can do away with you and produce a new creation.} [Quran 14: 19]
What is meant with 'a new creation'?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The meaning of the verse is that if Allah had willed to remove the people who existed when this verse was revealed, He would have done so and brought a new people in their place. The evidence for this is the saying of Allah (which means): {If He wills, he can do away with you and give succession after you to whomever He wills, just as He produced you from the descendants of another people.} [Quran 6:133], as well as the saying of Allah (which means): {If He wills, He can do away with you, O people, and bring others [in your place]. And ever is Allah competent to do that.} [Quran 4:133]

It is also probable that the meaning is to exterminate the existing people and resurrect them again; Ibn Katheer said:

“The saying of Allah: {If He wills, He can remove you and bring (in your place) a new creation. And for Allah that is not hard or difficult.} [14:19-20] means, it is not hard or impossible for Allah to do that. Rather, it is easy for Him, that if you defy His order, He takes you away and brings in your place others who are unlike you. Allah said in another verse: {O mankind! It is you who stand in need of Allah. But Allah is Rich, Worthy of all praise. If He willed, He could destroy you and bring about a new creation. And that is not hard for Allah.} [35:15-17] Allah also says: {And if you turn away, He will exchange you for some other people and they will not be your likes.} [Quran 47:38] Allah further says (what means): {O you who believe! Whoever from among you turns back from his religion, Allah will bring a people whom He will love and they will love Him.} [Quran 5:54] And (He also says): {If He wills, He can take you away, O people, and bring others. And Allah is Ever All-Potent over that.} [Quran 4:132]” 

Tafseer At-Tabari reads, “If your Lord wills, O people, he will destroy you, because He had created you without being in need of you; {and produce a new creation} [Quran 14:19]; Allah says: He brings another creation that will obey Him and abide by His order and refrain from committing what He forbade them from doing.”

Tafseer As-Sa’di reads, “It is probable that the meaning is: If He wills, He will remove you and bring a people other than you, who would be more obedient to Allah than you. It is also probable that the meaning is: If He so wills, He will cause you to be exterminated and then He will bring you back by resurrection as a new creation. What indicates this probability is what Allah had mentioned afterwards [after this verse] about the conditions of Resurrection.”

On the other hand, At-Tahreer wat-Tanweer reads:

“Bringing a new creation is used in bringing people who did not exist before, and whose existence was not expected, i.e. He would bring a people who will believe in Allah. Creation here meaning the creature, such as the saying of Allah: {This is the creation of Allah. So show Me what those other than Him have created.} [Quran 31:11] This is regarding the meaning of the saying of Allah: {And if you turn away, He will replace you with another people; then they will not be the likes of you.} [Quran 47: 38]”

Allah knows best.