Most people say that "dor" is one purely Romanian word as no decent translation has been proposed...
Is this correct: "Veți fi dor de noi foarte mult" ("You will miss us very much")?
No. "Vă va fi foarte (tare) dor de noi” would be correct. The 3rd person future tense of ”a fi” is ”va fi” ( present : ”este”)
As crbratu pointed out you need the third person because the construction is kind of impersonal: ”dor” is always the subject, so the verb does not change with the person(s) involved. You say ”îmi, ne, vă este dor” (I, we, you miss), and in the (literary) future tense this impersonal ”este” becomes ”va fi.”