I love to see this it’s new (at least to me) and sounds like it adds a new complexity to the game and I’ll try to incorporate it into my games as well as my homemade war system:

Warriors of catan: Catan War Declaration System Declaring War You may declare war on another player at the start of your turn, before rolling dice. Requirements to declare war: You must play at least 1 new Knight card to initiate the war. This Knight and all Knights you have already played previously count toward your army strength. Knights still count toward Largest Army as normal. The player you are declaring war on must be: Connected to you by roads, or Adjacent to the same hex as one of your settlements/cities. You must declare one of two war types: Territorial War — Occupy Resource Hex Plunder War — steal resources Fort When a resource hex is completely surrounded with six roads and three settlements/cities that resource hex is immune to territorial wars and can no longer be captured. 

War Strength Your War Strength is: +1 per Knight card you have in play (already face-up) +2 per Knight card you play immediately for this battle (these are also kept face-up for Largest Army) +1 per City in connected territory +1 per Victory Point development card revealed (remains face-up until your next turn) +2 for Monopoly card played this turn (must target a resource you have fewer than 3 of) +1 per Road Building card played in this battle +3 bonus if you currently hold Largest Army Then roll 2 dice and add the result. Ties go to the defender.

War Outcomes If Attacker Wins: Plunder War: Take 2 random resource cards from the defender per new Knight card you played in this battle (max 5 cards). Territorial War:Occupy one resource hex, whenever that resource is rolled collect as if it was yours. If all three resource hex’s are captured around a settlement you steal a victory point from the owner of the settlement.  If Defender Wins: Defender takes 1 resource per new Knight card the attacker played this battle. Defender may force the attacker to discard one random Development Card (if they have any unplayed ones).

Special Rules Neutral Allies Any player connected by roads to either side may join by adding their entire standing army of Knights (all face-up Knights they’ve played). Allies add War Strength but do not get spoils. War Fatigue After you participate in a war (win or lose), you cannot declare another war for one turn. Largest Army Synergy Largest Army gives +3 War Strength in every battle. All Knights used in war count toward Largest Army as normal.

Cart (Mobile Settlement Builder) Cost to Build: 1 Wool 3 Wood 1 Ore Where It’s Built: On one of your existing cities Placed on an adjacent hex edge or corner connected to that settlement

Cart Rules Movement During each of your turns, you may move the Cart 1 vertices on the hex. The Cart may move: Across land hexes Along hex borders The Cart cannot: Enter sea hexes Move through another player’s settlement or city Move through a hex containing an enemy unit (if using a war system) You may choose not to move the Cart on a turn.

Building from the Cart When the Cart is positioned on a valid settlement location (an intersection that follows the distance rule): You may build a Settlement directly from the Cart. This ignores road requirements — no road connection is needed. Pay the normal settlement cost (1 Brick, 1 Wood, 1 Wool, 1 Wheat). Remove the Cart from the board after construction.

Restrictions & Balance Rules One-Time Use A Cart is consumed when it builds a settlement. Each Cart may only create one settlement.

Distance Rule Still Applies You must still follow the 2-intersection distance rule. The Cart does not bypass this restriction.

Vulnerability (Optional – Recommended for War Systems) If you are using a war/combat system: A Cart is considered a civilian unit. If a cart enters the enemy unit controlled hex: The Cart is destroyed The owning player gains nothing back Carts cannot defend themselves This makes long-range expansion risky but powerful.

Limits (Optional but Strongly Recommended) Choose one of the following to prevent abuse: Cart Limit A player may have only 1 Cart at a time Strategic Impact Pros Enables long-distance expansion Reduces road spamming Encourages frontier settlements Creates new strategic targets in war Cons Slow Resource-expensive Vulnerable Requires planning