"It's going to be a completely new game." I lol'd. No it's not, it's in TN's best interest to keep everything the same with the exception of a couple of new characters and balance tweaks. They wouldn't risk making it shittier than how it already is because they would have to patch it again, again, and again. This is best shown by the support of DoA 4. It only recieved a few patches and it was never touched by them again. TN doesn't want to pay 40k every time they fuck up a patch and then patch that patch with another patch. That's a lot of money.
DOA4 was left alone because Itagaki was satisfied with it.