Take up 5* Aegon, Archangel, Void and Magik if you are looking for Act6 and upwards. Having already Magneto and Ghost, you will have a relatively easy task.
Definitely depends on whether or not you run suicide masteries. If so, rank up Corvus, Colossus, Sentinel as 6*, Aegon and Archangel as 5*. If not, all of the above are still good, but also consider Sunspot, Falcon, Tigra, etc.
Other: None. Act 6 will introduce a new way to use your resources: Rank what you need/what you're missing. You'll need a champion counter, a medusa counter, etc. Hold your resources until that thing arrives, then rank accordingly. None of those options are champs that you NEED at r2.
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Having already Magneto and Ghost, you will have a relatively easy task.