I had the pleasure of audiencing an interview with Ron Carter back in 86' and he gave a great technique which I use to date. It forces me to play more rhythmically and think more about the tune. He said that with all his students, when first teaching them how to form an intuitive supportive bass line, he limits them to the root and fifth of what ever chord. No meandering and BS passing tones justified after the fact. It's a good warm up to a tune- So far as etudes go, are you playing electric or a tree. For the tree I suggest the standard Bach cello suites. They've been transposed for bass. the trick for me was to not think I had to play them fast, at least until I could hear and feel it. Hope this helps and all.
Solo violin concertos are cool to arrange for bass guitar, especially 5- or 6-string since the violin has a slightly wider total range than the 4 string bass. Violin, like bass guitar, is a mostly monophonic instrument, but like bass it can play some chords, so the stuff tends to work. Unlike the bass guitar, violin is tuned in fifths. So transfering the music to an instrument tuned in fourths (bass guitar) makes for lots of scary leaps around the fretboard. Great practice. Cello music is cool too, except you keep running into that pesky tenor (alto?) clef. You really can't go wrong with Bach's solo cello sonatas, or solo violin sonatas. Paganini's solo violin music is wonderful virtuoso showstopper stuff designed to showcase awesome chops. Good luck.
I'm finding more and more that playing along with the radio is pretty fun, and helps me most with what I need to improve (Ear and spontaneous playing). I try to duplicate what the bass player is playing, and once I get that, then I embellish, or come up with my own bass lines.