Not sure if anyone reads forums nowadays but I would like to share my 2cents for this matter.
I truly believe before we move on to adding content we should focus on catering to our player base/new players. Over the last few months the population has never exceeded 50 to my knowledge.
4th job? new map? mules? leeching? These are good ideas because there's no right or wrong answers. Most of the people that play Maplestory private servers now are players from the 2000's era when Maplestory was in fact, like phoenix right now, 1x vanilla. But below its popularity was the community. it was the exchange of words, the adventure, and the bonding between players that made this game truly unique to its players.
3rd job vanilla is in fact the strongest selling point for Phoenix as Kradia said above, I think this should stay as is. What I suggest is either make the server 2x exp or do occasional 2x events for "x" amount of hours so that the casuals can join and have a feel of the game. What keeps the casuals away is the 1x exp aspect of the server. With occasional or permanent 2x people will be more willing to try the server because 2x is same as Maplelegends across the street. With other private servers filled with 4th job and even 5th job from GMS, phoenix advertising its 3rd job only will be putting something new in the sea of private servers. Other than that adding chairs and more cosmetics would be beneficial. Maybe dropping gachapon tickets (from mobs) that you can trade in for scrolls (10/60/100%'s) and chairs. This is pretty important because this will provide low level players a way to make income and motivation to train. Later on people will naturally cave to PQ's because its cost effective and provide scrolls. When we have secured player base we can add more pq's like pirate pq, romeo and juliet pq, etc. in the future. We just need something people can chew on to play the server.
What keeps a game afloat is the player base, any major changes like new maps or allowing mules or multi-clienting comes after. For the record, not allowing leech and multiclienting was a great move and it should stay as is. And I strongly encourage the admin to consider the possibility of chairs and cosmetics if not the gach drop.
Thank you for reading,