First of all before you even start thinking of the map you want to create, you must decide what type of map you want. You don't just jump in head first and start throwing **** around.. Well, that has never worked in my case. For me, I usually like to stick to symmetrical maps. (Warlock, The Pit etc..) When I make a symmetrical map, I usually start in the middle and work my way out. Default slayer in 4 is RvB (like all other Halo games) so two basses is a must, but keep in mind if you are going to play like multiteam you will need good places for the other initial spawn points and respawn points. (orange, green, purple etc..)
So just try this.. Make a tower of power style symmetrical map. Create a center location that is higher than the rest of the map. Make that location have the exact same thing on both sides. (red side, blue side) Then create two ways up to that location on one side. (there will then be four ways to the top mids) Now make both of those paths connect in one way or another to a base location. (I recommend using one of the "Buildings" with in the structures category; making your own base out of building blocks isn't really the best way to go..) Now make it to where you can get from one base to the other base by going around the center location without ever having to step foot on it. Just make sure everything is exactly the same on both sides and you have yourself a map! You can change that same logic around anyway you want and the flow of the map will still work.
Focus on one side of the map only. Don't even start building the other half until you are for sure how it is going to be. What I do a lot is just pop in player mode and jump, shoot, throw grenades, rape and fly around to figure out all the possible angles and how I should implement them better at that point in the map.
Anywho, I sure hope you read all of this!
Trollya later man!
But to answer your question, inspiration comes from my friend named Puff, the Magic Dragon. You know, the one who lived by the sea..?