I think getting mille for the RMR will be the safest for now since there are a variety of open emitter optics that will fit that footprint and probably will for the near future:
Trijicon RMR
Trijicon SRO
Holosun HS407C
Holosun HS507C
Holosun HS508T
Riton X3 Tactix PRD
NcStar VISM FlipDot Pro
Swampfox Kingslayer
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