Give audiences that futuristic flavor, and if a sequel is greenlit, then Cosmic Boy, Saturn Girl, Lightning Lad, Brainiac 5 and the rest of the gang can visit a different period.
Let’s start with a property where there’s no room for negotiation regarding its primary time period.
But for a Legion of Super-Heroes movie, keep the story set in the year 3,000 whenever.
It should be noted that the Legion does have time travel technology, so they can travel to the past or future if it’s required for a high stakes mission.