David Warner was definitely the best opener for Australia. The Australian batsman's ability to play in Test ODI and T23 format was excellent. He retired from Test cricket while in form and there are rumors that he will start his next career as a team coach. However, he has said that he will become a coach only if his wife allows him to become a coach. As much as David Warner has excelled in his career as a player, he will excel in his career as a coach. If he chooses coaching career then maybe Australia team can keep him as batting coach or as assistant coach but it's a matter of time before he has to decide to get license before he can take charge of a team.
Warner has retired from Test and ODI but will continue to play T20 cricket, as per my knowledge. He is no doubt one of the greatest opening batsmen produced in the history that can play test, ODI and T20 format. He play test as genuine test player while in T20I he is a solid hitter like a young player. Aussies have a good domestic setup and they will produce some other great opener to replace Warner.