I remember a game there was a kind of Happy Hour. When happy hour started, it changed temporary the game balance.

For example you could implement exp boost hours (50% more exp, 100% more exp "double exp", 200% more exp "tripple exp".

This is a nice mechanic to boost game moral of players. Think of a global message like this:

"Server : It's happy hour time. Exp boosted."
All players : yeah
And then gm launch happy hour again.
"Server : Happy Happy Hour - DDDDDDDDDouble EXP"
All players: YEAH
and then gm launch his final attack
"Server : Super Super Happy Happy Hour - TTTTTTTTTTTTriple EXP"
And all players: YYYYYYYEEEEEEAAAAAAHHHHHHH !!!

That's how you can motivate the masses. They want partys, like townraids.

Ok. Back to topic, you can also plan fixed happy hours on special days, like chrismas or weekend.

Happy hour could also be a kind of loot bonus (more luck or special drops).
Think of a very small chance to get normaly special monster bounded drops on all drops. Of course this chance must be very low and in relation to player level.

...

And then on the other site you can boost not the masses, instead you can boost the single player. Think of a rest bonus. One hour each day rested players have +10% exp bonus for example.

So you can do much things with more dynamic level system:

I looked your exp calculation ...

exp.c mostly line 474 you have included a bonus system for each player
Looks like bonus is defined not relativ, instead absolut. So it's 110% for (+10%) there ...

Didn't found a global boost, only a cap flag for capping max level to 1/4 (line 479) ...

So you could implent a new line, it's nearly copy the line 474, except you work with server_exp_bonus instead of player_exp_bonus ...

I can't say if a level rest bonus from +10% is to weak, but this could help to support normal players, with lesser time to play each day.

With enhanced exp mechanic you can also influence the whole game.
Players who reaching the end of exp boost, perhaps stops grinding for levels and go town, to do more human conversation things or clean there appartments, or only sitting there like in the good old days.

There is also the idea of boosting exp on player groups, in diablo 3 you gain more exp and loot luck for each player in your "group". Of course also monster get tougher for each player in group. It's *130% more life and +15% more damage there.

But back to topic. Some more ways to influence game balance from gm on running server like happy hours could be really interesting.

Hope i didn't say something you have implented before and i have currently not seen.