For some reason I started combining the board games Monopoly, Battleship, and Chutes and ladders.
Basically, it plays as Monopoly, but with those two kind of sewn onto it in a horrid, Frankenstein's monster of extra rules complications.
Simply put, it uses Battleship and Chutes and Ladders to offer extra playing fields and alternate win conditions. Each player has a Battleship field that starts out completely empty, and the chutes and ladder board is placed right next to the monopoly board with each player having a separate character representing their monopoly character on that starting place as well.
You play monopoly normally, if you land on Free Parking or Start, you can purchase a battleship for your side, with the smallest one costing 400, with each larger ship going up in increments of 200 (so it's 400, 600, 800, etc; destroyers and submarine's cost the same because they use the same number of pegs), and you have to buy the smallest vessel first before moving to the next largest. A player can only have one of each type, and is placed on their battleship board when it's purchased and can never be moved again. Unsunken ships can be bought and sold between players in normal trades, and this is the only way to move a ship from it's placed location. Damaged ships are permanently damaged and can't be repaired, and the damage is persistent when being traded from one player to another. You can't sell ships back to the bank.
For each ship you own, then each turn you can pay 25 bucks to fire a shot at a player's property of your choice. If it has a hotel or houses, it goes down by one housing level (if it had a hotel, it goes back down to four houses, then four houses to three and so on...) and if there were no properties on it, then it's owner loses ownership of it and it is placed back up for sale. That has the additional caveat that if that property was mortgaged when it goes back up for sale, it's previous owner needs to pay it back or immediately declare bankruptcy and lose the game.
However, if you can't fire on a player's properties if they own a ship of their own. Then you must face their ship by paying to take a turn playing battleship against that player normally. This has the complication of needing graph paper or something so each player can keep track of each spot shot, which will only be rendered obsoleted again and again as players purchase new ships.
A player successfully sinking another player's ship earns a war prize equal to double that ship's value from the bank. The destroyed ships are left on that players board, however, they can purchase a new, duplicate ships to replace it. This may require a large number of ship tokens given an extended game.
If, at any point in the game, a player has a complete set of still floating ships and each other player has no ships, that player declares a military victory and wins the game automatically.
The chutes and ladders board represents a character's spiritual progression. In addition to a player's ordinary monopoly move, they can decide to be charitable and forgiving, and donate 25 dollars to any opponent to take a spin on the chutes and ladders wheel and move your character's spiritual avatar there forward following the rules of chutes and ladders. However, you can't move your spiritual avatar if you have a net worth (value of all property, including ships and houses + cash on hand) exceeding 2000 dollars. The wealthy can't attain enlightenment so long as they're weighed down by their worldly possessions afterall.
If a player gets to the final space on the chutes and ladders board, that player transcends humanity and obtains a spiritual victory, winning the game instantly.
To accommodate for these changes, the Chance and Community Chest cards would be altered to include things that get rid of boring stuff like "You win a beauty pageant and get 10 bucks" and include more relevant things like "Keep this card, and you can use this card to negate a ship attack" or "Keep this card. Use it to take three spins on the spiritual wheel, disregarding current wealth" and you could trade them and use them as bargaining chips in potential trades.
I just thought these would be fun changes and I wanted to commit them to writing, though I'll admit that it might lengthen a normal game of Monopoly, which already have a bad reputation for taking a long time, to requiring even ridiculously longer stretches of time for a group to sit down and just play a board game.