General synonyms:
* Indeterminate
* Unsatisfactory
* Uncertain
* Ambiguous
* Unclear
* Doubtful
* Unconvincing
* Unresolved
* Open-ended
More specific synonyms:
* Not definitive: (for results or conclusions)
* Not decisive: (for actions or outcomes)
* Unsubstantiated: (for claims or evidence)
* Insufficient: (for evidence or proof)
* Undecided: (for a decision or outcome)
The best synonym for "inconclusive" will depend on the specific situation and the nuance you want to convey.