5. That isn't possible without changing the source code of openttd itself, as GameScripts doesn't allow it, and we'd like to just use GameScripts. - so unless it's going to change in future openttd releases.. it'll remain like it is. (you can buy a water tower)
As for diamonds, it's the same issue as above, it's not possible with GameScripts.
6. I just adjusted it today, try a few games and see how it is now.
7. You can't block anyone from doing that, but we are working on a way to punish people when they do so.
8. See previous reply about aircraft/airports
*snip*
By the way I remembered more suggestions :
5.Some towns does not have water tower. Every town should have water tower and bank and diamonds should be a requirements for town to grow up like passenger good and food.
6.Mail requirements are too high, it should be a little bit decreased.
7. (I noticed lucky) building station or company building to another company's town should be disabled. I dont know how but I tried built bus station in middle of another's town and it worked.
8. (I know I just said it) Make able aircrafts. Limit could be 2 or 3 but it needs to be able.