I agree that the schedule params should be consolidated. I don't think it would be bad to coalesce around schedule_interval instead of making a new one. Cron expressions are already not really "intervals", and I think that asking the overwhelming majority of DAGs to change a param name is likely not worth it. schedule_interval already takes two dissimilar things, cron expressions and timedeltas; we should just keep adding things that it can take: Timetables, event rules, and whatever comes later.
That said, getting to one param is far more important to me than if that is a new param or an old one that gets extended.
