"idea 1" is correct and usable in written English.
For example, you could use it as the title of a section of a written document, such as "Idea 1: Suggestions for Increasing Employee Engagement".
Idea 1: Hold 2-3 open farm days, invite homeschool groups by email, Facebook, and fliers.
Idea 1: You want to shift a significant portion of your savings into progressively more conservative investments as the date of your retirement draws nigh.
A couple of newsworthy items concerning our neighbor to the south (Not including Karin's "Google Gate Bridge" idea): 1. Google Earth gets assisted by new satellite; 2. Google Earth used in Fossett search.
Big Idea #1: Computers perceive the world using sensors.
We call idea 1 PARAFAC-NTF (p-NTF).
Idea #1: Assemble over 600 naloxone rescue kits with the North Carolina Harm Reduction Coalition.
Recent discoveries of UCP1 orthologs in fish and non-Eutherian mammals have now discredited this idea [ 1, 2, 17], as fish and most non-Eutherian mammals do not contain brown adipose tissue where UCP1 plays a major role in thermogenesis.
-- Stockholm: International IDEA, 2008.
A simple idea 50 years in the thinking..
Robert Moses, his descendant, conceived the idea 14 years ago.
They were not even a new idea 30 years ago.
