Some ideas for level selection...

[Kapitany]

I think a scrolling shooter where the player can "travel" from one level to another can be an acceptable idea.

Or a Terra Cognita (http://www.mobygames.com/game/terra-cognita/screenshots) remake with different levels should be a good choice.

[Eric]

Will going back or replaying the locations (without selection) count as revisiting? Or should the locations be player-selectable to count for a revisit?

[WILL]

Will going back or replaying the locations (without selection) count as revisiting? Or should the locations be player-selectable to count for a revisit?

Absolutely. You can do either, however to keep the world open and explorable, you'd have to have distinct locations within that world so they would be best to define them rather than leaving it up to the judges have to count them. As the rules state, they must be all distinct with their own visual style. So in the case of Castlevania 2 style play for example, you'd have to create 3 distinct looking areas or locations.

Example 1) You have a forrest, a village and a cave for example. You labeled each by having some text showing above when you arrived in each area.

Example 2) You have 3 different looking cities or towns, but the path connecting each is all forest or just something common to each. Again when you got to the town or cities a text label above or a city/town sign at the entrance showed that you were at it.

The idea is to define 3 locations which each have their different visual theme and figure out how to travel to/from each. How you go about it was left open so long as you have at least 3 locations minimum.

[Ñuño Martínez]

I've remembered another nice game that uses "travel": Kirby and the Amazing Mirror. This video is long but it shows the concept with a lot of different places:

[SilverWarior]

Can I asume that multiple star systems would count as multiple locations since theese star systems differ from each other by the number of planets, planets types, etc.

[WILL]

Can I asume that multiple star systems would count as multiple locations since theese star systems differ from each other by the number of planets, planets types, etc.

Well see there is somewhat of a challenge with free-roaming 3D space too. You would, just like 2D, have to define that location either visually or by indications (labels, shown on a map and such). You could have a 'warp drive' that allows you to warp or jump from location to location, or auto-pilot, etc, but then there is the challenge of being in one location and decorating it in 3D so that when in another location you decorate that one so they look different from each other.

That's the main work of the theme depending on how you design your game, make locations that are "visually" different from each other which you can do other things to enhance that already visual difference. Different music alone doesn't count as per the rules, it has to look different.

Now having different planets does do a little to help define a difference, but I'd go a bit further than that and give more than just the planets as a feature. Maybe an asteroid field, different coloured space dust, a few nebulae and have your 'HUD' tell you you are in ____ system or something. That would do it I think. The scores will somewhat reflect how well you design towards the theme too.

[SilverWarior]

Actualy if I'll have enough time I will include planetmaps aswell. This should definitly count as different locations becouse each planet will have its own texture representing its surface. And becouse there wil be various types of planets it would be perfectly clear that those a truly different locations.
But for now I don't want to get ahead of myself.