I'd rather err on the side of not letting another army just universally negate such a huge chunk of mechanics by handing out Fearless.
Still... hmm, maybe some sort of buff. I suppose I could up Leadership, particularly on champions. A universal special rule boosting their ld tests for certain things is another option.
Yeah, that sounds decent. Up the cost of the marines by 1 point (13 points for basic CSM, thematic) and give them a special rule that lets them do the old 'roll three dice, discard the highest' thing for morale and Fear tests?