You can create any systems you want, but a few things about systems to help out.
Firstly the scenery. It helps to have a really nice looking system. The more attractive the better (note - not cram it full of stuff, just make it look nice)
Its gotta be well thought out. As Giskard says, draw out your system first - it really does help!
Functionality - what is it for? Fighting, trading, exploration or general playability?
Think a little before building. Manhatten has tons of bases, Omicron Gamma has 1. I prefer to play in gamma as manhatten is boring. Its great for a starting system, but dull. You don't need tradelanes, but they are nice if flying alot - depends on location of your system really. Bases - think you WHOSE they are, why are they there? No point putting a GMG base unless there is something that means GMG are there for......... same with the pirates, they all have a reason for where they are..........
Best thing i can say is build the base of the system first - star, light source, jumpgate/ hole entry. Then when you save this (i build mine as a mod - so i can do this part easily) make a backup! I cock systems up all the time, or if my changes look like sh*t etc, i can delete and just unzip my file again and start with a working system etc. Keep doing this as you build the system in stages you are happy with, its quick and efficient.

Last thing - populations, encounters and missions - you gotta have them obviously - but missions - hell yeah. Remember - what is oging to make someone stay and play in your system?
Hope that helps you get started - those are the things i think about - what will keep them there - does it look good, what on earth can you do there etc

Don't ask what players want in a mod, think of what YOU want in your mod. For instance, I live in in the American state of Connecticut. Since it seems en vougue for American system modders to base a Liberty system on their home state, I would contemplate adding such a system to Liberty space.

Since I was an avid Privateer fan many years ago, I might also contemplate an Independant world system called Gemini and load it up with some of the bases and characters from the first game.

I also have a slightly bent sense of humor, so I might want a new Corsair system which has a planet called Morocco, orbited by a moon named Gibralter, then put one small station called "Barbary Post" in the system (insert pun dmage here).

I could go on and on and on with ideas for creative initiative. Get the editing tools, play around with some xperiments, but release nothing until you decide you have what *you* want.

I put the "fun" into "dysfunctional."

Chips, thanks for posting this... made four new systems .