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You can vary the ostinato however you like, provided it's still recognisable. Altering the rhythm is orthodox, as is just implying the ground through the choice of scales and chords. Switching to something totally different is unadvisable; in the above example Bach actually uses the ground as the subject of the fugue - that's how determined he is to exploit the motif to the full.
I suggest you forget playability for the time being and work on composing something that *sounds* idiomatic. If anyone asks, it's now for mechanical organ. Mozart's most famous piece was composed for just that, and I daresay baroque composers would have done the same had they been provided with the opportunity.
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