The idea for protect bank is a bit OP. Maybe if it were to protect the most valuable stack in your bank (so you only lose the 9 most valuable stacks) it would be a bit less overpowered.
The bank dropping to the floor with 5 keys thing is another idea I like. Another thing that would be good from the seasonal to go to normal DMM would obviously be the GE.
As for the quest idea, ruins the whole fun of questing! I don't think that would fit good in DMM original.
As for the last idea, I think that would be amazing. DarkScape was sooo much better than DMM in my opinion. For DMM to change to be more like an OSRS version of DarkScape would be a lot better, but I don't think it would pass 75% on a poll.