I can respect that reasoning... though, then there needs to be two stages beyond BF, PS2 and SV, and that's kind of what this topic ends up arguing around, doesn't it? most people agree on those three being good starters, but it's evening them out with two more that's an issue
if horizontal space is why that isn't good enough on its own, what vertical-based stages would work well? that way, one would have a middle ground in BF, two horizontal stages scaling from small to extreme in PS2 and SV, respectively, and... honestly, I'm stumped from there. WW could fit the extreme for vertical, maybe?
There is a huge problem with PS2, SV, and BF involving blast zones. PS2 and SV both have low ceilings and fairly close blast zones. This means vertical killers and characters with poor recovery would benefit heavily from this stage list. Any floaty would automatically have a bad matchup against characters like Fox who rely on strong finishers on stage. Characters like jiggs and peach that rely on their strong horizontal recovery and high blast zones to survive would be massively disadvantaged. Furthermore, the three maps listed all have bottoms which are short making characters who use wall jumps in their recovery suffer. This is not ideal. So lets look at other options to add for a 5 stage list. I will address my views of how they address these problems and rate how well they address the problems mentioned above. (Feel free to skip this part if you just want to get to the point).
I feel like GHZ's high ceiling is good for balancing out the slightly low ceilings of PS2 and SV, though it's side blast zones are still close. It does give at least one starter to characters that like wall jumps to recover. Personal Rating 4.5/5
Yoshi island's blast zones are similar to PS2, so having it as a starter is kind if redundant. However, it has wall jump recovery options, filling that void. I love this map, but it doesn't really fit into neutral. Personal rating 2/5
Lylat is actually a pretty decent idea for it's high ceiling and decent side zones, though not many people like that map. However, it fills the gap nicely. However, it is probably the worst stage for characters who use wall jumps to recover. Personal rating 4/5
FoD is a lot like GHZ in that it has a higher ceiling and close blast zones, but I feel most people like GHZ better. It is a nice middle point for wall jumping though since it doesn't let a character do it all the way from the lowest part of the blast zone. personal rating 3.5/5
One that few seem to think about as a possibility is Metal Cavern, but as far as the blast zone problem goes, it is essentially a poor man's Dreamland. It's ceiling isn't nearly as high, but the side blast zones are about as far and it's ceiling is still medium height. It also isn't as big as dreamland so it isn't as much of an auto win for floaties. Personal rating 3.5/5
FD is perfect for blast zones, but it's lack of platforms make it's gameplay pretty polarized so I understand removing it. However, doing so hurts both the blast zone and wall jump problem. It has the most balanced walls because they concave under the stage making the wall jumps harder, but still useful than maps like GHZ/YS where the walls are vertical. It has problems but the way it addresses the problems of the other maps make it pretty useful. Personal rating 3.5/5
Wario Ware is probably the worst idea for addressing the blast zone problem because it easily has the smallest blast zones in the game. Personal rating 1/5
Rumble falls has a high ceiling and medium side zones, so it addresses the blast zone problem, but the stage itself is so big and the platforms on top are so tall that run away camping is too easy. Personal rating 2/5
Distant planet as close blast zones and a low ceiling. However, it is a large stage, so it isn't as bad as other close blast low ceiling maps for addressing the blast zone problem and it has a good wall jump platform. Also gameplay is awkward. Personal rating 1.5/5
Dreamland is the opposite extreme from the current problem so adding it back for a more balanced list isn't that bad of an idea, but I think there are much better options, and due to it's advantage for floaties it isn't very neutral. It is a great stage, but it benefits floaties too much. Personal rating 2/5
Yoshi island (B) still has low ceiling, but it's side zones are far which helps a lot in the balance. It's main detractor is the clouds that come from the bottom to save people. It also has a wall for wall jumping. IMO, it is a better stage than people give it credit for. Personal rating 3/5
Skyworld has a high ceiling and medium side zones, making it good for the blast zone problem. However, it is generally hated by most and it's still bad for wall jump recoveries. Personal rating 3/5
Skyloft has a low ceiling and medium side zones. It also has a pretty frustrating layout. Personal rating 1/5
DK 64 was a neutral back in SSB and is also a poor man's dreamland with the blast zones. However, even without the barrel on the bottom, this map is super easy to camp out. I think it would work better than the melee version but still not good. Personal Rating 2/5
PS1 has the same blast zones as PS2 pretty much and has other problems. It really shouldn't be considered. Personal rating 1/5
Dracula castle is pretty decent. It still has a low ceiling, but it's side zones are far and the stage is fairly big which helps balance the size of the other stages. It also has walls to jump off for wall jumpers. Personal Rating 2.5/5.
Idk how Norfair 2.0's blast zones are, but it might be a good choice since it is basically SV with two platforms ins stage layout. Also, it has a nice medium for characters that like wall jump recoveries since there is a decently sized wall on the bottom. Personal rating right now assuming blast zones are both medium 3.5/5
(end of breakdown)
I think that if PS2, SV, and BF are guaranteed to be neutrals, that the best maps to be neutrals to compliment them would be (in my personal order): GHZ, Lylat, FD, FoD, Norfair 2.0, Metal Cavern, Yoshi Island.
That being said, I don't think that most people will agree because of the fact that those maps aren't necessarily the most popular maps in the game and I don't think these are the "best" maps. I just feel that with the biases toward close blast zones on PS2 and SV, that two of these maps should be added. Personally, I would go with GHZ and Lylat even though I hate Lylat. However, if the lack of neutrals with wall jumpable sides is deemed important enough, or the hate for Lylat is strong enough, then either FD or FoD should take the second spot. Also, if Norfair 2.0 ends up being amazing when it comes out, then it should take the second spot. Metal Cavern and Yoshi Island are more shots in the dark than anything, but they could hypothetically work.
To be clear if we were to use 7 neutrals, then maps with close blast zones wouldn't be as significant so the rating for wide blast zone maps would go down and the rating for close blast zone maps would go up. The ratings above assume that there will only be 5 neutrals. If there were 7 neutrals, maps like YS and Wario ware would increase in viability and maps like dracula castle and DK 64 would decrease in viability.
That is just my opinion though. Feel free to tell me any flaws in my breakdown.