i agree with shorter times between defenders (15 mins is a bit long) but i do agree with chait on swapping pokes, I can think of an exception tho like after a time limit being able to swap a poke (24 hours)
Chait my point is that if we take out the time limit people can defend their gyms for more time like can include 21 pokes at a time and can defend it. Keeping this in mind the ones trying to attack will have trouble so they try to bring up their clan mates so which becomes a war.
Now comes the real thing the clan who defends the guild for a long time gets longer rewards.
Like if a clan defends it for consecutive 3 hours they win sure gold or great money like 1m (just saying)
5hours= 5 gold and 2 m
10 hours = 10 gold and 3m
and this must be distributed automatically according to players who have participated.