That's not to say that a pure Flying Pokemon can't exist in the future, but like others said, it would have to be designed as such so that it can't learn or use Roost in any way.
Or they just change the mechanics of Roost.
I don't know if we will ever see a pure Flying type nonetheless. We've gone 4 generations, 65 part-flying Pokemon, 493 Pokemon in all - and no pure Flying type.
For what it's worth, out of 153 possible type combinations, 88 have been done, 65 not done. So even among 'type combinations', pure-Flying is in the minority in not existing.
On the plus side - 65 type pairs not yet done is plenty of room for new Pokemon.
EDIT: I had one concept for a Pokemon that would be ???-type. Not sure if I posted it already (I may have not done so on the grounds it's a bit rubbish).
'Infrabolus" is the embodiment of chaos and the counterpart to Arceus. The name is based on the Latin for lowest+devil, as a parallel to Arceus' possible etymology being highest+god. Stress is on the second syllable, similar to 'parabola'.
680 BST, likely offensively oriented. Its tru type is ???, but its unique ability, "Chaos Flux", causes it to randomly change typing every turn, to any of the 289 possible 'ordered' type combinations (17 of those would have primary and secondary the same; they would be treated as pure-types not as 'double' pure types.).
Its signature move would be 80 BP, physical (to counteract Judgement, which is special), and typing would be Infrabolus' primary typing at the time of use. It would get a generally large movepool, but perhaps lack the most powerful moves, on the grounds of chaos being unable to be focussed to such power, for if it was focussed it would become ordered.