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Why not have Meso Bags? You can store mesos in them like how you store ores/plates/jewels in Mineral Bags.

And make the bag tradable? Else there would be no point making it except having the ability to store more mesos in a character, you'll still need to trade multiple times anyway.

Why go through all those troubles when you can just simple increase the cap?

Simple. Change it from a Signed INT to an Unsigned INT. That one bit doubles the capacity to 4.2B

That'll cause more trouble in coding the other part of games. I'm pretty sure a lot of actions that take away your mesos are using negative values.

Code it just like how they code the hp of monsters currently, and give it a hard-coded cap, as said above, should be better.