It can be done pretty much literally by changing one number, it has the least potential complications and has to my understanding already been tested with no meaningful complications.
There is a DoS attack vector at 2 MB that can be malicious exploited, especially by a mining cartel. It seems like you are heavily uninformed.
You may think it scales linearly, and that's not really a good idea on its own.
Signature hashing is quadratic not linear.
I was talking about the blocksize thing. I like the signature thing for many inputs to the same address, that actually makes sense.