In the formation of a marketing strategy, it is imperative that one consider/considers the marketing mix, also known as the 4Ps – product, price, place, and promotion.
You can use either version, the one using "consider" or the one using "considers". Both versions are grammatical; and for your sentence, both versions have the same meaning.
LONG ANSWER: For simplification's sake, let's just look at part of your sentence--the part in italics.
- it is imperative [that one consider/considers the marketing mix]
In your example, the adjective "imperative" is a mandative word that can license a subordinate subjunctive clause (see CGEL, page 999, [15.i]), which in your case would be the declarative content clause:
- (that) one consider the marketing mix
That above subjunctive clause is headed by a plain form of a verb, which here is the verb "consider"--that is what makes it a subjunctive clause. (Note that the plain form of a verb is used in many constructions, such as the infinitival, imperative, and the subjunctive.) In this case, the mandative construction is a subjunctive mandative.
You could also use a, er, regular-sort-of declarative content clause--that is, a non-subjunctive clause--such as:
- (that) one considers the marketing mix
where the present-tense verb "considers" is in agreement with its subject "one". In this case, the mandative construction is a covert mandative.
There is also a should mandative:
- (that) one should consider the marketing mix
The above are the three main subtypes of mandative constructions: subjunctive, covert, should.
NOTE: Here is an interesting excerpt in regards to AmE vs BrE differences in usage. From the 2005 textbook by Huddleston and Pullum, A Student's Introduction to English Grammar, pages 176-7:
. . . This should mandative is more common in BrE than in AmE, where the subjunctive mandative is generally preferred.
NOTE: Also, here is an interesting excerpt in regards to the health-and-well-being of the subjunctive in today's English. From the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language (CGEL), subsection "Distribution of mandative clauses", pages 998-1000:
It should be emphasized, however, that there can be no question of giving a definitive list of mandative items: in spite of suggestions that have been frequently made that the subjunctive is dying out in English, this construction is very much alive, with attested examples like I would stress that people just be aware of the danger suggesting that its distribution is increasing.
Here are two relatively recent answer-posts that deal with the mandative construction:
Though, be aware that the term "subjunctive" is used in various different ways. Many still use it in ways that traditional grammars have used it, and some modern grammars use it in a more restricted way (which is the way I usually try to use the term). And so, don't let the different usages of that term mess you up too much.
