My guess is that he was thinking about crystalline cohomology.
It fits rather nicely in Grothendieck research at that time. He had obviously in mind the success of Dwork's p-adic approach to the Weil conjectures, and the limitations of the other cohomologies avaible at the time (see sections 1.5 to 1.8 of "Crystals and the de Ram cohomology of schemes").
The standard conjectures were worked out in 1965 (according to Grothendieck's 1968 paper), so he had to have them in mind while working on his p-adic cohomology, that he presented to Bourbaki on december 1966. He then gave it to his student Pierre Berthelot to develop. The intro of the Bourbaki notes reads:
The content of the notes are by no means intended to be a complete
theory. Rather, they outline the start of a program of work which has
still not been carried out (*).
(*) For a more detailed exposition and progress in this direction, we
refer to the work of P. Berthelot, to be developped presumably in SGA
Berthelot's complete exposition was not presented as SGA 8, but as in independient work in 1974. So even if the cohomology was ready long before that, Grothendieck had to regard it as unsolved in 1968, when he wrote about the standard conjectures. It is also reasonable to imagine that he had hopes at those early stages for crystalline cohomology to be an important tool in the yoga of motives.
Again, this is just a guess. I'm not sure that he would refer to this as "unsolved geometric questions" (perhaps in the sense of its aplication to Hodge/Betti coefficients?). Maybe someone who knows more about all of this can add some details.
Some relevant references:
"On the de Rham cohomology of algebraic varieties" (Grothendieck, written in 1963)
"Crystals and the de Rham cohomology of schemes" pp. 254-306 (Grothendieck, written in 1966)
"Letter to Tate" (Grothendieck, written in 1966)