Thank you for reviewing my plugin. These are great suggestions and I'll add them to my todo list.

I've worked to try and make the payouts very balanced/realistic. Similar to a real slot machine; this pays out ~95% of the money that is paid in. I've been playing around with how it's calculated (if you have a suggestion please share).

Currently, each type of block is assigned a base reward-amount (0.2 (dirt) -> 1.2 (emerald)). I then search the 2D array for blocks that are connected (left to right, top to bottom, diagonally). For each connected pair the following is added to the payout:
(weight * bet * offset) / 2.
The offset is a number I added to manage the total payout percentage (currently 1.419).

For a single pair of connected cobblestone when betting 100:
(0.3 * 100 * 1.419) / 2 = 21.285.
Having a full row or column of cobblestone would be:
(0.3 * 100 * 1.419) = 42.57.
Having a full row or column of emeralds would be:
(1.2 * 100 * 1.419) = 170.28

In the future, I plan on making the payout mechanism more extensible and allow for customization. The plugin already has a few config options, however, they're a bit ambiguous and I plan on making a video to explain their function.

P.S. I had sound added into the plugin, however, I removed it at the last minute before this release. I play the game at a low volume and a friend that was testing the plugin alerted me to the fact that it was extremely loud.. Once I figure that out sound will be added back.