I think I read one similar suggestion before, but I think its lost, somewhere. Unless I forgot it was me.
Max level : Infinite
1% bonus from plunder per level.
1% cheaper goods per level (0.01% goldbars)
Cost
100 million (25 million increase per level)
500 credits (100 credit increase per level)
1000000 guild xp (500000 xp increase per level)
You can donate gold to the guild to be used for upgrade.
You can donate credits to the guild to be used for upgrade.
Guild earn 1 xp for every goods sold.
Guild earn 250 xp per battle won.
Guild lose 200 xp per battle lost.
Balance maybe off, unless tested.