Throwing more battery at the problem may not be the best solution. However, improving the energy density of the battery would go a long way towards improving battery life. Having something like 6400 mAh in a battery the size of the Nexus 4's would greatly improve battery life, and allow them to use more high performance, energy hungry parts.
I would like to see a 5" Nexus phone WITH a very slim bezel. They could keep the overall size of the phone about the same, and have more usable screen area. However, I wouldn't call a 5" screened device a phablet. A lot of manufactures seem to be releasing highend smartphones that have screens about that size.