What about this:

After certain research you get an option to hire ships from the black market. You get a option where you can select how many ships of each type you would like to have and for what amount of time you want them or maybe just what amount of fleet capacity the fleet should have (like RA where you have no option to select the exact ships). Of course it would get more and more expensive the more and the longer you want them. After paying your fee the ships spawn somewhere (hmm where... your homeworld? the pirate base? a selected planet like with a returning armada? maybe this should require some kind of black market building you have to research and build... whatever). The difference to RA is that these ships DO NOT add to your used fleet capacity. You can use these ships for a certain amount of time - there should be a maximum of course or the ships should get increasingly expensive witn time so that you cant just hire them for 60 minutes or so but just for some minutes. Then they disappear like the extra frigates you can spawn with the duplicate forces ability. This would be great for eco players who need some defence in their inner planets when being sneaked or if someone needs some extra firepower somewhere. Of course this service sould be well priced as having these extra ships could add a big advantage to the player who called them.

Ok, would this be useful? If so, how exactly should it work? Should it require special buildings as beacons? maybe use an envoy cruiser as target - finally a good reason to hunt and kill these ships as they could call pirate assistance behind enemy lines. Also the prices need to be balanced. However I think this adds huge new possibilities to the game and would allow  lot of new surprising strategies.