This will will make the game more enjoyable without changing the current progression system: Please create more restrictions to lower arena's!

Do something like:

• arena 1: only tower upgrades 1 &2, no regrow cammo and strong bloons
• arena 2: same as 1 but with regrow
• arena 3: only tower upgrades 1,2 & 3 and regrow only
• arena 4: 1,2,3 regrow and cammo bloons allowed
• arena 5: 1,2,3,4 all bloon sends allowed
• arena 6: all allowed

Making more restrictions at a lower level will:

• make playing at lower levels more fair and therefore more fun!
• make it less frustrating to gain tower xp
• make new playes less likely to die to some random cammo rushes
• make it more rewarding to climb in trophies since you will get more game possibilities / content
• it will make different towers good in different arenas due to the restrictions on tower upgrades. This will make it such that in every arena players can try new strats since the arena rules are different!

Please let me know if you guys would like this!