When trying to determine the best TO-BE architecture, it can be useful to describe future scenarios. Each scenario can be described by the 5W2H method : Why ? What ? Where ? When ? Who ? How ? How much ? (QQOCCP in french).
You can then discuss the pro/cons as well as the obstacles and pre-requisites of each scenario. Once you know the preferred scenarios, it’s easier to draw the target (TO-BE) architecture.