pre-noun adjectival
ideal; desirable; the way something should be
Pre-noun adjectival (rentaishi) expressing an ideal state or what ought to be. Often used in phrases like あるべき姿 (ideal form) or あるべき姿に戻る (return to how things should be).
会社のあるべき姿を議論する。
We discuss the ideal form of the company.
彼はあるべき姿に戻ろうと努力している。
He is striving to return to how things should be.
理想 is a noun meaning 'ideal' as a concept or goal, while あるべき is a pre-noun adjectival describing something as being in its ideal state.
望ましい is an i-adjective meaning 'desirable' or 'preferable', often used for actions or outcomes, while あるべき focuses on the inherent ideal state of something.
Derived from the verb ある (to be) in its attributive form ある + the classical auxiliary べき (should/ought to), literally 'that which should be'.