Well, first of all I think tiers should also unlock new stuff – weapons, ships and other items.
I’d also like to mention a feature from another game that, if implemented correctly, could possibly make things a bit more interesting.
So. In ShellShock Live 2 there is a feature called Prestige. It works as follows:
After a player reaches level 50 (which is the maximum level in this game), they have the option to prestige.
If they choose to prestige, they get reset back to level 1 (though they do keep all of their earned XP) and all weapons obtained through levelling up are locked again (though they don’t lose their upgrades – after reaching the weapon’s XP requirements it is given back to the player in its upgraded state).
As compensation, the player recieves a prestige weapon (which, as you may expect, is meant to be pretty damn powerful).
The player can prestige up to 10 times, receiving a new weapon each time.
The general idea is that the prestige weapons will allow the player to earn XP more quickly and reach level 50 faster next time.
This system was replaced in the Steam reboot by a simpler levelling system (with levels 81-100 now labelled as prestige and indicated by golden numerals).
Now, of course Chicken Invaders isn’t designed to be a competitive PvP experience, so the system would require some tweaking.
So to summarise:
After reaching the highest tier, the player would be given the option to prestige.
If they choose to do so, they will be reset back to tier 1 (whilst keeping all the score they earned). If taking away the items unlocked through tiers is decided not to be a good idea (which it probably isn’t), a different temporary handicap could be implemented.
In exchange, the player is awarded a special, otherwise unobtainable item (weapon, spaceship etc.).
This item is meant to help the player earn keys and score at a faster pace and generally make the game easier.
The player can prestige a finite amount of times, receiving a new special item each time.