Hopefully there's a new Chalice build path in the works. It has potential to be the big new Siege item, but currently there's no build path a siege build mage would really want.
You could build Grail, but unless you're Lux, Orianna, or "Mage Morgana", you cannot use the additional passive it gets. At all. You just stack up blood charges and they sit there for the rest of the game.
You could build Mikael's, but that'd only be a decent choice if the enemy has snares or hard slows abound, as you cannot cleanse yourself if you cannot cast spells.
Beyond that, well, if you had the gall to actually enjoy a siege build, you're just SOL.
Ugh, first they finally pronounce Dominion (finally, tbh) dead, and now this. Sure, Fun is on a weekly weekend rotation, but a lack of siege build options kinda just kills my will to play at all.