Great composers are not measured by the amount of ideas they have but rather what they do with a few ideas. Very often, young or inexperienced composers tend to write pieces that play out one idea just to move on to another idea etc. while losing a continuity in their piece and making the composition feel like a Medley. However being able to sustain an idea for a longer period and still keep it interesting is what makes a great composer.
Probably THE most well known example where this strategy is obvious is the 1st movement of Beethoven’s 5th Symphony. The key motif (3 short notes followed by a long note) is not what Beethoven gets praised for. Actually that idea is so simple that probably everybody could have invented it. However what makes Beethoven a true master is the fact that he bases a whole ca. 6min movement practically on this very motif alone without getting boring for a second.
That is management of ideas to the extreme but the essence here is that he managed to make it seem effortless to maintain a momentum in a (almost) 1-motif-piece. So whenever you feel the need of moving on to a new idea in a piece because you feel like you’ve exhausted what could be squeezed out from your initial idea, think about Beethoven and if you could make your piece more stringent by sticking to the initial idea a little longer.
By the way, the argument of “this is classical music, what do I care” is invalid. The Raider’s March from Indiana Jones by John Williams is practically a one motif piece as well as long as it stay’s in Indy’s theme (this time the main idea is dotted eight followed by 16th followed by a target note/figure).