The BIG problem with long time ress investmant is that Ai war is a game where you can take yourselve some time to build up. so every stukture can be build quite in a mass and so bonusses become stronger and stronger till they lead to a turtl tactik where they player sits 1-2 houers only whatsching his econemy build up and doing nothing else.
So long term investmants shoudn't give you more ress or more energy or more ships, but other things ^^.
Perhaps in the econ Tap in the build menue ther can be a Logisticcenter, what has no cap costs
300K / 300K / 0 Energy some build time and hase a % modifyer making it only giing good bonuses at 10-15 building in the system, and making you engeniers have a better Pionir rate, lets you ships travel faster in systems you own, making you engeniers jup though all your systems that are connectet to help repairing, give you the abilety to build train stations for yous transports to take then into shuttel servise, and so on..
perhaps all 5 building 1 new Logistic breaktrough, when your Techs in the buildings manage your empire a bit better.
at the down side the AI shoud get 50 Proces for each killed Station, and an X Modifier on the waves aganst the system where X is the number of these Station in the system, and a special wave all 2h aganst the system to shut the Logistics down. that comes as CPA when the system has no mor connected AI systems but in this special CPA the AI get the ships from the wave span Pool and dos't use the standing around ships.
When the Game gets more Funktions we choud implement all the controll Nodes in such a Long time to build Stukture, so you buil your Empire but althou your Ligistik Struktures up that allouws you to get sometime some realy helfull skills for you Empire.
ok Cost are to discuss, and how many skills you get erly ^^ and what are the Skills everyone whant and we can hide behind a 50+ Stations wall so you realy don't get them till latest game.