i am a bit in map making and a lot in clan games, so basically i have thought a lot of on this theme. just my 2 cents:
the map layout - two general paths to objective, but one of them dividing in two close to obj. (imagine them as I and Y schematic view). The shortest path would be the simple one, but much easier to defend. It is used for rushes when a whole team tries to take the defenders by surprise or when u need to get quickly to the obj after wiping out most of the defence. The second path is were u go usually - it is longer but rather safe as defenders dont see your exact movements. at the end right before teams meat ofenders can choose which way to go as the path divides in two. Between these two should be some structures that block visibility so that its impossible to defend them both from the same position with ease (like 2 floors, 2 doors on different sides of the building). Examples are good old rtcw maps: base (exclude radar 2 from layout - noone defends it), the final stage of beach, were u carry objective etc.
To alter the leghth of those paths u can use 2 spawnpoints for offenders.
spawn times should be in intervals 20 and 30 seconds. other nominals make it hard to calculate every next opponents reinformenttime once u spotted one. If one team has 30 secs and other 35 secs its a disadvatage for 30 sec team as its hard to calculate when u have to be ready to face reinforcements. eaqual spawn times might give an unfair advantage for one team too. An example would be the defenders respawning always some seconds before offenders get to the place they can start attack. As a result defenders are always at their positions and ready to defend. That kind of game often can produce full holds. Its best to have 30 sec spawn for defending team and 20 sec for offending team to my mind. This approach also makes it possible for offenders to attack in stages - e.g. to wipe out enemy primary defence, hold the place and wait for reinforcements.
size of the map should not be large. large maps are fps killers and noone likes them. rtcw base was a perfect example of 1 obj map (without a flag), beach, ice, village - 2 obj maps. One or two obj is what u need. more is too hard to adapt.