Well here goes
As for first game and for testing here's what i propose
Climate: sub tropical
map: 512 x 512
industries: medium
citys: medium
TL 7
no boats
50-100 planes
lots of trucks/trains
newgrf's: Long vehicles
goal:world population, like alot, maybe 2m to start out, (openttdcoop managed over 3M in first, 6M in second try in 512x512)
playtime: 1970-goal is reached(or if found goal unreachable) in general, since we dont use autopause, the game should take about 24h to reach goal.
since we all use the same company to reach common goal, but to somewhat give individual challenge aswell, each player could take a city, or two to develop and look after. simple renameing so people would understand who is looking after that city. the common goal is to get world population up, but this mini goal, to see whose city will develop to biggest will maybe add a little extra kick to the game.
some of us maybe don't like to citybuild, don't you worry. theres also a need to produce cargo, food, goods....but all that should be shared, so in my dream world it goes something like one-two place to produce stuff, and then deliver them out evenly. theres no point to deliver all the goods to one city whyle others need them to grow aswell. so even if you take it as some kind of small contest to build your city to biggest, then keep in mind the common goal of world population, your city alone cant reach it, so share the stuff.
And for god sake, use signs. co-op is like programming, you need to comment your code for other person to understand. same goes here, sign the purpose, the drop stations, pickup stations, plans, and so on, so if someone log in in the middle of game, he could easily understand what is going on and where and what is needed to be done. etc you need to log out, leave a sign that "need more grain to be delivered" so the next player can see that and do that.
Also share the built out network, theres no need for new line for something, if theres existing line which isnt used to its full capacity.
Waypoints help out alot and also use shared orders
PS. maybe locking out entire server isn't the best idea, let there be spectators who can see and become jeleous and feel the urge to join the community, but to get in the company, only selected people.