you can still construct a custom deck using 60 or more cards, which must be from just one or two of the games different colour-coded ink types - similar to mana colours in MTG
Umezawas Jitte Back in Betrayers of Kamigawa , they printed a piece of equipment that basically did everything you could want in combat, and then some
An XRR spell is a long way from mana efficient, it never really gets to being efficient either, even when it does double damage
Using creatures such as Mnemonic Wall, Archaeomancer, and Ardent Elementalist, you can cast one of these flicker spells on the recursion creature to be able to get your flicker spell back instantly
It isnt a direct replacement for Phyrexian Altar, but it is a neat little backup with its own set of advantages and disadvantages
This effort is a multi-year series [of releases]