My primary concern wasn't LOS = death, it's LOS = information. In a game where positioning is paramount, knowing where your enemy is going is critical. CS has other differences such as smaller map scale and fast player movement to account for CTs often not knowing where the Ts are going until they're contesting an objective. As this relates to your proposed mode, I'm worried that defenders will--on open maps--be able to react to the attacking team's composition & movements well before the attackers have begun contesting an objective, therefore making the objective itself no more than a MacGuffin as the rounds would almost exclusively be decided by the outcome of a full-on team fight instead of destruction of the objective. If you're going to have an objective-based mode, then playing the objective must always be a viable strategy. If defenders get to fully position themselves before the attackers can begin contesting then the attackers no longer play for the objective, they play for kills.
Ideally, defenders should have to split their forces between the two objectives in such a way that they can delay the attacking team without risking too many deaths, giving their teammates time to rotate. The attackers then should be focusing on tying up the first defenders and securing the objective. The rest of the defenders should show up before the objective can be wholly secured, making the attackers choose between finishing the objective or diverting all of their attention to fighting. Naturally, maps with greater distance between the two objectives should have longer sight lines to give the defenders earlier warning. The off-site defenders should always arrive at about the same time to a contested objective unless they adopt a strategy other than wait-and-see (like scouting for attackers with cav & ambushing from a central point).
Ideally, defenders should have to split their forces between the two objectives in such a way that they can delay the attacking team without risking too many deaths, giving their teammates time to rotate. The attackers then should be focusing on tying up the first defenders and securing the objective. The rest of the defenders should show up before the objective can be wholly secured, making the attackers choose between finishing the objective or diverting all of their attention to fighting. Naturally, maps with greater distance between the two objectives should have longer sight lines to give the defenders earlier warning. The off-site defenders should always arrive at about the same time to a contested objective unless they adopt a strategy other than wait-and-see (like scouting for attackers with cav & ambushing from a central point).