If you dont know their gate address you could still attack them, you would just need trasport ships, or an upgrade from somewhere.
However, if you did attack through space, it would take mabye a turn or 2 to get there, which means that you cant just steal their naqadau because they are trying to buy an upgrade. However, you would still be able to do damage or steal naquada from accounts just building up the naqada.
This may be a little off topic, but you could also introduce more than 1 planet per person, meaning there would be a couple of gate addresses per player, (possibly the other planets resource planets or something) and reintroduce the raid feature for miners working on the resource planets. Untrained mercs, a use for them
. I dont know, the planets would also have to be conquerable and some way of introducing them to the game, either finding or getting them once you get certain levels of rank strike, defence, covert, etc. But that sort of upgrade would take ages to program.
All in all though
, would be hard to introduce though.
oh and mabye you could get someones gate address if you defeat the in an attack or another special covert feature, or both.