Don't release just a simple scenario or a playback, which even average Orbiter users can create in 10 Minutes. This will not be cool. Cool are things, which not everybody can do, which show that you have either skills or spent more time into making it as the average Orbiter user would.
I would recommend you, to make add-ons for yourself for the first time, most developers started just like that. The first add-on I released, was actually already number add-on number 8 for me, and only few of the earlier add-ons had been good enough to become released after serious work. My personal quota is roughly like that: For each add-on I released or each add-on where I contributed to, there are about four experiments and add-ons, which had not been good enough for release.
What would be considered cool by many people in the Orbiter world, would be somebody making the famous landmarks of Earth as bases or additions to bases. For example the great pyramids of Giza, which are even visible from LEO.
This is not a overly complex job, but requires creativity, basic skills with making good meshes with a small number of triangles and finally, a good eye for the essence of the object you model.
Usually, a base is a good start for newbie developers, as you don't need to learn physics or fiddle with physical parameters. If you release a base, don't forget putting one or two scenarios in to the add-on package.
