The big expansion, War of the Chosen, already dropped, so I'd say it is safe to pick up.
Although I don't really get this way of thinking. The major DLC's for both Xcom's basically rework the structure of the game. Playing Xcom vanilla and then playing through it with the Enemy Within expansion are two fairly different experiences. It is the same way with Xcom 2 vanilla and the War of the Chosen expansion.
edit: Bummed that Phoenix Point is being pushed back, but it is looking like a huge project and I can see why they would need more time.