I like
Isaac, though I've never heard it pronounced so "zac" is recognizable. I used to know one and he never went by any nicknames; very nice, well-mannered, kind-hearted kid; would be in college by now, I think.
Of your list, I best like
Isaac Julian and
Isaac Arthur. I like the feel of
Isaac Gideon but the "c" to "G" transition is a bit sticky for me. Please excuse any repeats of ideas...
Isaac BartholomewIsaac BeauregardIsaac BertramIsaac BonifaceIsaac DonovanIsaac EvanderIsaac FaramundIsaac Fitzwilliam
Isaac FletcherIsaac NevilleIsaac ReuelIsaac Romero
Isaac VernonIsaac Willem