You could still get your wish; it's just that Janet wouldn't be the daughter of Maria or "Janet Sr." (The age difference wouldn't work out anyway: so far all the cast rumors for Janet -- or the female lead, in any event -- is in the 30something range, making her DOB in the 1980s, not the 1960s. Or earlier.)
And yeah, I like the idea of Maria being in this 1960s first reel. If she's a comrade and/or Significant Other of Pym, and winds up getting brutalized and murdered by Castillo in a callback to the actual Astonishing Tales era, that'd be a great reason for Pym to retire from the spy game and spend the rest of his life bitterly blaming SHIELD's politics for her death and his own failures/shortcomings.