i agree that trade is one of the weak points in GC and ive been toying with dufferent solution based on a pyramid scheme style trading system
for the system i envision
generating trade routes
each planet would have the capability of sponsoring 0-2 trade routes(outgoing) based on planet class (loosely tied to planet quality),but could have any number of incoming trade routes.
profit and maintenance
each trade route would provide a fixed amount of profit ~1bc to each of the planets and would cost an amount equal to 1bc per ~20 tiles distance
pyramid scheme
when one planet is trading with multiple other planets each trade route gets a 10% bonus to income based on the number of unique trade planets it is trading with
ex Planet A has 4 direct trade routes to planets B,C,D,E in this case the trade routes each earn 130% of their total amount. this would result in 1.3BC credits for each of planets B-E and 5.2 BC for Planet A, a total of 10.4 BC with Maintenance costs of 4BC assuming all planets are within 20 tiles of planet A
adding in secondary trade routes will add a 5% bonus taking the above example lets add planets F, G they are trading with planet E
this would increase A-B/C/D trade routes to 140% each (the original 100% +3 primary trade routes at 10% and 2 secondary at 5%) the A-E trade route would increase to 150%, and the two new trade routes would be at 135%. assuming all planets are within 20 tiles of each other this would result in 16.8 BC profit and 6 BC maint
tertiary trades and all additonal routes thereafter
would add 1% expanding on our current pattern (B/C/D - A - E - F/G) if we add 5 more planets to G, H-L
A-B/C/D would increase by 5% each (1% for each of the new tertiary trades) for 145%
E-A/F would increase by 25% each (5% for each of the secondary trades) for a total of 175% (E-A) and 160 (E-F)
E-G would increase by 50% (10% for each of the new primary trades) for a total of 185
G-H/I/J/K/L would each produce 173% (5x Primary 2x secondary 3x tertiary)
this would cause 36.4BC per turn profit and 11BC per turn Maintenance
unique trades
each trade route would only be counted if it provides a unique trading partner in my example (B/C/D - A - E - F/G - H/I/J/K/L)
if planet H started Trading with Planet F it would increase its trade by both planets by 5% instead of 10 % since they would both gain a Primary trade route (+10%) but their secondary trade routes with each other through the E-G trade route dont count (-5%)
trade hubs this system would generally encourage you to build trade hubs 1 planet in your empire with which all other planets are trading you would most likely only use secondary trade routes to connect planets that cannot support their own trade routes or to avoid large maintenance costs from isolated groups of planets
you would also be encouraged to find trade hubs from other civs and connect them to your trade hubs