If interplanetary trade is added, it could be similar to the GBT. The GBT in its current state is overran with massive offers and the like, so what if it was broken up. The GBT could be a much later building by needing starships. It could also take time as opposed to being instant. It could be broken up into sections based on a towns level (1,2,3) or a trade level. This would let massive trades be contained to people who can actually pay them, while leaving small/new towns with smaller trades.
If this forum isn't active, then ill make it active!