slightly different advice from above
drums
drummers tend to actually do the variation thing every even bar so bars 1, 3, 5 and 7 are exactly the same and bars 2, 4, 6 and 8 have variations
it is also common to have a crash come at the end of bar 4/start of bar 5 and definitely at the end of 8/start of 9
a drum break may not be the best solution every 8th bar, especially if you go OTT with it
bass
bass players tend to follow the kick for major points of emphasis: these will be chord tones with chord tone 1 being prevalent on beat 1 and beat 3
if they use more notes than there are kicks in a bar these will be passing tones (tones between chord tones) or neighbour tones that return to the previous chord tone - the note a 2nd above or below the active chord tone.
bass players will also add some variation that complements the drums where they vary: think octave leaps or snaps from 7ths into octaves to add emphasis to the drum parts
keys
a keyboard part can be static and simply hold the active chord for the length of each bar, or it can be active and move so that it uses similar ideas to a bass part i.e. passing chords/tones as you move through inversions of the current chord (use partial scale runs, either harmonised or not, to connect each inversion) and neighbour chords/tones: decorative parts that emphasise the current chord by accentuating the "rightness" of that chord by moving to it from "wrong" chords/non-chord scale tones