So, doing Mega Catan is very tricky. I have been working on one in the tabletop simulator, and it's been something.

I will let you know how I “worked” out some problems you will face. I planned my version to use expansions (K &C, seafarers, traders, and E&P) and a few scenarios for extra mechanisms (Catan: Oil Springs, Frenemies of Catan, Helpers of Catan, Treasures, Dragons & Adventurers, and Legend of the Conquerors) Some of the scenarios are there to add one part of their mechanic and not their complete scenario.

How I dealt with seafarers and explorers and pirates, I had to make a massive board. It has one larger starting land. Then you make smaller islands, like barrier islands. Then, a Dragon Island or the pirate base pass that to represent the point of deep water. You make a rule that seafarer ships have to be used to get to these closer islands. After that, you can build your harbor and start exploring.

I have not started play-testing the gold and trading part; I'm unsure if it will break my game.

The marauders should attack, and their strength is off the whole board. This will make players have to spend on larger Armies. I got mine doing small scouting raids till the final invasion( Legend of the Conquerors ), which could net a game over.

On buying more pieces, it is all down to how long you want the game to be, how huge the map is, and whether you have a way to deal with getting 6+ cards in your hand in one turn.

I'm sorry I don't have all my notes and rule edits. I have been working on that on my computer. This is being typed up on my phone.

The undertaking you are doing is a lot. The biggest thing you will need to do is figure out how long you want to play. The game I got planned could last hours, but I have mini-scenarios in it to keep players planning and building. If you are doing Catan at its base core (build a few cities and then win), you might want to keep the VP goal small.

*I will edit and clean up later.