Two good answers already for the example in the question, by aCOSwt and Laure. The title, however, appears more open and general than the one example in the question.
For another possible usage of “down the road”, where something specific is predicted to happen at some unknown future moment, though its potentiality is already present or imminent, we could envisage à un moment donné (also à moment donné in some areas of the francophone world) or tôt ou tard (not particularly metaphoric, but I tend to agree with Laure’s statement that there are not many colourful equivalents in French, beyond “l’eau sous les ponts” already proposed by aCOSwt):
- If we leave it this way, it will come to bite us down the road.
Si on le laisse ainsi, ça tournera/se virera contre nous à (un) moment donné.
Si on ne s’en occupe pas, ça nous retombera sur le nez1 tôt ou tard.
Obviously, there’s a proverb for almost anything, so one could perhaps cite one, but it will need to be adapted on a case-by-case basis and likely won’t ever be a general case. But it is one way to transpose an image without falling flat in translation...
For the previous example, it could be some prediction of dark ages:
- Qui sème le vent récolte la tempête. On ne peut pas laisser ça tel quel.
In another type of use, it could simply be a call to inaction:
- Don’t worry, things will improve down the road regardless.
T’inquiète ! Après la pluie le beau temps !
1 From the common image “cracher en l’air pour que cela vous retombe sur le nez”.