Can I choose my chosen champion in the champion zone with primordial roar ?

Radiance rules aren't published yet. This uses the printed card and current rules only, and it isn't a ruling on the new mechanics.

No — you can't pick your Chosen Champion from the Champion Zone for Primordial Roar's Show Off. Show Off on this card is "reveal a unit from hand or pick a friendly unit," and the Champion Zone is neither of those: it's a non-board zone, so a champion sitting there isn't a unit on the board and can't be chosen by effects that choose units (Rule 355.9.a.1; FAQ #1213). You'd have to play the champion to your Base or a battlefield you control first.

One timing note: the Show Off choice is made as the spell is played/finalized (Rule 355.5), so you can't pick the champion and then try to play it in reaction to make it legal — the pick and the damage are locked in before anyone can respond.

This is my interpretation based on Rule 355.9.a.1 and the non-board status of the Champion Zone. No official FAQ exists for this specific interaction.