I believe that a good way to encourage more people to play often is to have an upgrade (leveling system) cross-server. This is because people are naturally completionists who want to accomplish goals.

EXP would work roughly like this:

EXP gain for CTF = 400 for winning or (Points / 2) for losing + 10 * (Flag Captures) + 2 * (Human Kills)
EXP gain for Infected = 250 for winning + 2 * (infected kills) for human, and 60 for winning + 10 * (Human kills) for infecteds. The last human surviving gets +75 exp.
EXP gain for Boomo = 25 * (Player Count) for winning or 5 * (Player Count) for losing + 1 for each powerup collected
EXP gain for Halo = 200 for winning or 100 for losing + 5 * (Human Kills)
EXP gain for Spleef = 100 * (Player Count - 2) for win and 50 * (Player Count - 2) for losing, minimum of 3 players to gain any reward since spleef is very exploitable with only 2 players.
EXP gain for UHS = 25 per minute you survive without going AFK. Maximum of 15,000 EXP per UHS life.
EXP gain for KitPvP = 4 * (Kill Streak)^2 EXP, with a killstreak maximum of 50 and EXP is rewarded when you die.
EXP gain for SkyBlock = 10,000 - 100 * (Island Rank - 1) per week. (Island Rank - 1) means that the top island gets the full 10,000 exp reward. This encourages people to play skyblock more.
EXP gain for SkyWars = 250 EXP for winning or 50 EXP for losing + 25 * (Player Kills)
EXP gain for Parkour = (Meeble Award for Finishing) / 10 + 100 * (Checkpoint). This reward is doubled in Chill Room.
EXP gain in Event = 1,500 for winning or 100 for losing
EXP gain for Jobs = 10 * (Job Level) each time you level up a job. Ex: Level 49 Woodcutter would give you 490 EXP, and then level 50 Woodcutter would give you 500 EXP.
EXP for Towny = 1000 - 25 * (Towny top (Residents -> town) rank) + 1000 - 25 * (Towny top (Residents -> Nation) rank) for all towny or nation members per day, and Mayors/Nation Owners get 3x/5x the EXP respectively.
EXP for Wild = 200 for each Supply Crate

Quest NPC

The quest NPC will offer 10 daily quests which reward 1500 EXP and 500 Meebles for each quest completed, for things such as CTF challenges or other specific tasks. These tasks should be heavily focused on towny things though, in order to promote the Towny aspect of meepcraft
The NPC will also sell EXP. The cost per EXP will be 20/ea and is limited to 10,000 EXP per day.

The Actual Level System

EXP is used to increase an overall Meepcraft level which is seen on either a scoreboard (like the ones you see in minigames) or with /meeplevel.
The formula for the level System would be similar to something like this: (Current Level) = 15 * (Level)^3. Level 1 = 10 EXP, Level 10 = 15,000 EXP, Level 20 = 120,000 EXP etc.
The minimum level = 0 and the maximum level = 100.

EXP gain Modifiers

Ranked Players will get a bonus multiplier based on their rank. The EXP gain increases by 25% per rank. Legends will obtain +0.75 instead of +0.25 to their modifier to help them level up faster.
Players can buy EXP boosters for $9.99 or $19.99 which will increase EXP multiplier for all players by 1.0x or 2.0x EXP respectively. This boost lasts 1 hour.
Each donation made for non-Booster items (Ranks, Heads, Fireworks) will give all players online at the time an EXP reward equivalent of (100 * Dollars Spent). The donor will also get this reward.
Occasional Events should occur where EXP gain for certain tasks is increased

Level Up Rewards

Level 5: Ability to do /near with a cooldown of 30s
Level 10: +0.25x NON-EXP Reward for Minigames
Level 15: Voting Reward: 2,000 -> 2,500 Meebles
Level 20: Unlocks /fix with the limit of 1 /fix per tool and costs $2,000 per usage of /fix
Level 25: More lenient Shout Taxes: 3 Shout Max -> 4 Shout Max, Shout Timer 10 Sec -> 8 Sec
Level 30: +0.25x NON-EXP Reward for Minigames (+0.50x)
Level 35: Access to /fill with a 1 Min. Cooldown
Level 40: Access to /feed (Minimum of 8.5 food bars)
Level 45: Access to /heal with a 2 Min. 30 Sec. Cooldown
Level 50: Shout Tax: 4 Shout Max -> 5 Shout Max, Shout Timer 8 Sec. -> 6 Sec., Tax - 25 Meebles
Level 55: /feed is increased to a 9.0 food bar minimum
Level 60: +1 usage of /fix per tool, Cost lowered to $1,500 per usage. (2 per tool)
Level 65: Cost of "Buy only" items in /warp shop decreased by 5%
Level 70: +0.5x NON-EXP Reward for Minigames (+1.0x)
Level 75: /fill no longer has a cooldown.
Level 80: Voting Rewards more likely to give Bonus Meeble rewards
Level 85: Pwarp cost and upkeep is lowered by 50%.
Level 90: +1 usage of /fix per tool, Cost lowered to $1,200 per usage. (3 per tool)
Level 95: /near has no cooldown, +0.5x NON-EXP Reward for Minigames (+1.5x)
Level 100: Infinite /fly
[COLOR=#000000]Any EXP past level 100 will be displayed but will not count towards any levels.[/COLOR]

This system was thought of by me within the span of a few hours only, so it's very rough and could likely use some heavy balancing and such. However, any EXP leveling system which gives decent rewards and perks will increase the player base actively playing and the player base as a whole.

Any feedback is appreciated.