You can simply say new company.
For example: "I will be starting at the new company next month."
Prospective employer is any employer who is considering a person for employment.
Therefore it may not only mean somewhere you will be employed but companies you have gone for interviews for.
New comapnyということができます。
I will be stating at the new company next monthと例えば言えるでしょう。
Prospective employerとは、雇用をする人のことを表します。
そのため、自分が働いている場所のみならず、インタビューに行く会社のことをさすこともあります。
Firstly, it is not a 'prospective employer'. A prospective employer is a possible or potential employer - for example if you have yet to attend an interview and be offered a job. The job is only a prospect, not a fact. If you 'will' be working for a company, you are no longer a prospect. You are a future employee.
You may speak in terms of the workplace, the employer or the employment.
Eg. "John future workplace is situated near London."
Eg. "John's future employment is near London."
まず「prospective employer」ではありません。「prospective employer」は「employer（雇用主）」になる可能性がある人のことです、例えば、仕事をオファーされたがまだ面接を受けていない場合。その段階ではまだ「prospect（見込み）」であって、「fact（事実）」ではありません。
もしその会社で働くことが決まっているなら、もう「prospect」ではありません、「future employee（将来の社員）」です。
「workplace（職場）」「employer（雇用主）」または「employment（職）」について言えます。
例）John's future workplace is situated near London.
（ジョンの内定先はロンドンの近くです。）
例）John's future employment is near London.
（ジョンの内定先はロンドンの近くです。）
Example
"I went to my new company today."
or
"I visited my future employer today."
or
"I am going to be working here in the future, this is my future employer."
"I went to my new company today."
今日、内定先に行ってきました。
"I visited my future employer today."
今日、内定先に行ってきました。
"I am going to be working here in the future, this is my future employer."
ここで将来働きます。私の将来の就職先です。
▪Future employer.
future=likely to happen in time to come
▪Considered company
considered=believe to be
▪Soon-to-be employer
soon-to-be=planned or destined to have
"Potential employer"
" An impending employer"
"A prospective employer"
"Potential employer and impending employer" can be used as alternate terms to describe a person who has received a job offer and will be joining the new company in the near future. "A prospective employer" means any employee who is considered for employment. "Prospective candidate can be used to explain future employee.
南アフリカ 
アメリカ合衆国 
イギリス 
日本 
