The at least one faction of that superpower already present rule is to stop PMF's aligned to a superpower starting wildly out of place, ie Alliance PMF in the middle of Imperial space etc. So you want to be looking at the top of the bubble (ie vertically upwards 100+ Ly from Sol on galaxy map) for candidate systems where alliance factions are prevailant.
The system you choose could make or break your faction, so I would advise paying close attention to the political situation in and around the systems you look at.
Search any candidate system in
Inara and check to see if there are any existing PMF's (or NPC MF's with obvious player backing if have expanded to multiple systems) already present in or in neighbouring systems. Does the area already look crowded with most if not all systems nearby already belonging to other PMF's? If you start in such an area then you are going to find it hard to find systems to expand to where you are not going to be opposed.
Also you want to check traffic levels in candidate system, if a system averages a lot of traffic then you are likely have a lot of random cmdrs working against your faction. A system that averages lower traffic means you will have an easier time.
The economy type and position of the main starport in the system should also be a consideration. Ideally the main starport which will be your factions HQ to work from should be close to the star, or else if it is a long trek in supercruise to get there you and your companions are likely going to get bored of having to do that all the time as well as will take longer to complete missions taken from the station. A high tech or industrial economy will mean the starport's shipyard and outfitting will be better equipped compared to other economy types and the economy type is also going to change what type of missions are available.
You also want to think about the population size of the system, low population size >100k means it will be very easy to to increase your influence in the system but just as likely to lose it quickly and be exposed to volatile swings in influence that can cause conflicts frequently. Where as a high population system 1 Billion+ means you will have to do a lot more work to get a foothold in the system but it will be a lot more stable with influences changing much more slowly. Personally I would make sure any candidate starting system is somewhere inbetween has at least 20M population and if you want to be able to takeover the system and pump out expansions relatively quickly no more than 500 million.
Once you have found some candidate systems, I would fly there and spend some time there in game to get a proper feel for the system and surrounding area to make sure you are picking the right one.