Yep, it's a real concern. Slingshot at least didn't hurt too much, IIRC, so it all depends on how much they gut existing code, rather than just add on to it.
From what I've seen of how the intrinsic code is structured, their previous DLC plans were only going to be the structured council mission campaigns and cosmetic add-ons (hair/helmet, face and deco armors). These can be added pretty easily without broaching the existing code.
Adding some new weapons wouldn't be too much more difficult for Firaxis (as opposed to us modders). Adding a new weapon requires creating a new child class of XGWeapon and adding it to the library (each child class is pretty lightweight), and then linking it to the new animation / mesh which is stored as a separate file. This also wouldn't be a huge impact on the code.
They may also re-add / finish off some features that didn't make it into the original release. There were tunnel swarm and street swarm missions as well as a base defense mission type. Adding/finishing these is going to mean more substantial changes to the code.
Creating new plot / mission objectives is going to mean some pretty substantial reworking of XComStrategyGame.upk. If there's a new campaign I'd say that this has to be in there.
Offhand I'd hazard to say that they really don't need to change much about the tactical game, so the XComGame.upk won't be changing as much as the strategy game upk.