Have you heard a lot of their Repertoire? There is a TON of Debussy that is much much much easier than the Fantasie Impromptu. I would say most of his Preludes don't have a severe technical difficulty (there are the obvious ones that have some tough spots).
For Liszt, probably one of the better entries to his rep is probably the Liebestraum (famous one). It's the first piece by Liszt I played, and I got introduced to his Cadenza writing, his harmonic language (progressions especially; and remote keys!), and the legendary octaves you see in nearly all of his pieces... I used to think this is easier than Fantasie Impromptu, but it does have some tough spots on its own (the climactic middle spots).
Ravel... he has more difficult rep than Debussy, and less easier rep to be honest. I hadn't played any Ravel until I had played some Debussy and a bit of Liszt (I started with the Sonatine, and then played Alborada). There are some pieces by him that are a bit easier (Pavane pour une Infante Defunte - which is still tough! Don't underestimate it, it becomes dense and it is very tricky placing all of the voices and harmonies without spamming the pedal)
I would suggest these by Debussy:
-Suite Bergamasque (The prelude and menuet are my favorites by far, but the entire suite kicks ass)
-These Preludes: "Minstrels", "la Cathedrale Engloutie", "le Vent dans la Plaine", "Ondine", "Feuilles Mortes"
-The "Sarabande" from the 'suite pour le piano'
-Masques (one of the most interesting pieces I've ever heard, because the initial motif is just way too catchy for its own good)
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The 'Bigger' pieces come with time, just like the 'bigger' pieces by any other composer... I'm sure there are some Mozart pieces, or Beethoven, or Bach that you probably look at now and say "no way", or your teacher would say "no way"...whatever you do, make sure not to put certain composers on this unreachable level and avoid them. Debussy, Ravel and Liszt each have accessible pieces, although they do have well-known pieces that ARE very tough...
Before playing Debussy, I 'enjoyed' the piano, and was just playing a few pieces at a time; after getting into it, and seeing what the piano could actually do (other than bug me each year because of RCM exams) really made me develop a new interest. So he's definitely my favorite composer, and a few years after getting into his stuff, nearly all his pieces have become accessible.
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Liszt - Check out the Liebestraum, or his 3rd Transcendental Etude (not frightening! It's a very beautiful piece with lots of texture and layers).
Ravel - not joking, you could try out Jeux D'eau. I had tried a bit of this pieces probably a bit too soon, but it got me really looking into different Ravel stuff, and creating a genuine interest in his stuff. Also check out the "Prelude" from 'le tombeau de couperin', "Oiseaux Tristes" from 'Miroirs', or "Pavane pour une Infante Defunte".
There's tons of good stuff out there! Have you checked out Rachmaninoff or Prokofiev??