Could Syria's Peace be Extended to all Parties?

Now that the Syrian opposition and the Bashar al Assad regime have accepted the United States(US)-Russia plan toward a ceasefire in the five-year-old Syrian civil war, did the bargaining powers forego an opportunity for full peace by not extending an olive branch to all the warring groups in Syria?

When the clock strikes midnight across Syria on Friday, a cessation of hostilities is expected to begin in the Syrian conflict. However, no militia designated as a terrorist group by the United Nations(UN) Security Council, is not covered by the plan. 

Therefore, the International Coalition fighting the Islamic State(IS) and any other terrorist group, could still continue attacks upon these groups. But in the name of a wider peace in Syria and in the region in general, could all groups have been covered under the ceasefire? Did the bargaining powers miss an opportunity to test any cooperativeness and any willingness to peace within these groups? All opportunities to achieve full peace in any conflict should always be considered.