if you are still on the lookout for original ideas to go into your game why not go to: http://forum.shmups.com/forum/
everyone there are shooter mad and will have a plethora of suggestions for you.
i dunno if your idea of the game never ending but continual random levels is a good one. i think people need to know they are striving for something, being a shooter addict myself i know that score is a damn good reason to play a game but others like to play to get that 1 credit clear. Perhaps two modes could work, a never-ending mad mode and a mode whereby once level xx has been reached you get congratulated and given a special upgrade or something and then the game goes into loop 2 which is basically the same as starting again only harder enemies or whatever.
if that doesnt quite make sense i can elaborate further if you are interested in the idea