I recently noted that Article 120(12) states: "Until the Commission has designated, pursuant to Article 27(2), issuing entities, GS1, HIBCC and ICCBBA shall be considered to be designated issuing entities."
and per Article 123(3): (i) Article 120(12) shall apply from 26 May 2019.
So, looks like if the EU hasn't designated issuing entities by 26 May 2019, then GS1, HIBCC, and ICCBBA are the issuing entities (of which at least GS1 and HIBCC are also for US, meaning we can use the same GTIN.