MTR 3.4 defines proxy cards as something a Head Judge creates, not something you bring from home
For example, you can setup Solemnity + Phyrexian Unlife to prevent you from dying or even fetch Solemnity to pair it with Decree of Silence to prevent your opponents from playing spells
But in my early evaluation, three color pairs stand out as underwhelming: white-black, white-green, and black-green
For 3WUBRG you buff your creatures based on the number of towns you control
(2018-03-16) If the triggered ability divides damage or distributes counters among a number of targets (for example, the ability of Bogardan Hellkite), the division and number of targets can't be changed
If an effect exiles Summon: Esper Maduin and then instructs you to return it to the battlefield, it remains face up in exile (unless that effect instructs you to put it onto the battlefield transformed, in which case it returns as Summon: Esper Maduin)