I personally think Sylvanas will make a good Warchief. She may be doing questionable actions now but I feel like they will build her leadership as the xpac goes on. She may be dark, but she will keep us alive and fighting, not dying for the sake of honour (not reference to anything that she done yet, but potential future events).
Plus, if they are really setting us up for ANOTHER warchief then Blizz needs to fire their writing team. That would just show poor and unimaginative writing.