I frequently find I can come up with a verse and a chorus structure but then adding the extra bits of song structure to give it some variety (intros, bridges, middle eights) gets a lot more difficult. It's generally around then that I start having regrets about my complete absence of music theory. :(
I rarely struggle to come up with ideas, the top lines aren’t down to music theory, arrangements are though.
I tend to put a section on loop and then drag individual notes of a chord to different inversions (CEG becomes EGC or GEC) or move one note at a time within a chord up or down a semitone to get augmented or diminished chords, the same with adding an extra note on a basic chord CEG for example added A, A# B etc.
The basic chords are usually obvious from where your melody starts and ends on each phrase within the melody i.e. first and last note (bottom and top) then if it’s a sad song the middle note will be three semitones from the bottom note and four for a major. Sorry if I’m teaching Granny to suck eggs here.
I sometimes realise I’ve got two separate song ideas that can, with some tweaking, become one song. I’m doing one like that now.
My real problem is getting distracted by new songs and not finishing the old ones. That’s mostly down to not having yet replaced guide vocals with well performed vocals, improved the drum programming from basic time keeping drum tracks, less of a problem now that I use Drummer in Logic, and in some songs I need to spend some time getting better sounds on midi parts. Then it’s down to spending time on the mix downs.
For your intro’s etc have you tried reversing the notes you used in your verse or chorus or don’t use a top line, just the rhythm section (drums plus chordal instruments) of the first few bars of the verse or chorus?