I'm not sure if they announced a time period for arcarum EX but i'll assume it's going to be featured in the livestream.
By that time, all the xeno raids will have come out so i expect they'll announce the plans for them as well. The current format of "replace an actual event with a xeno" is not very fun and i hope they add them to a rotation like their normal counterparts. Normal Ifrit, Cocy, etc. Also started as events so i'm holding out hope.