I can think of a number of justifiable reasons why an ISD-I(Chimaera) set is a real possibility and not just a waste of an expansion :-
1) The ISD-I differs from the ISD-II in a number of details enough to justify a new model/sculpt. (Eg. Bride detail, Tractor beam array, broadside cannons, forward dorsal cannons, engine fins, whiter painjob, Chimaera motif)
2) There has never been a toy/model/miniature of an ISD-I. Every Star Destroyer that has ever been made into merchandise is of the ISD-II variant so this is a great opportunity for FFG to be the first and possibly only manufacturer to make it.
3) A good chance to introduce specialised variants; command variant with a fleet command slot and a dedicated batteship variant(Tector) that trades engineering and squad value for more armor and firepower.
4) New titles and Commander(Thrawn, Chimaera, Executrix, Harbinger, Sovereign, etc)
Finally, canon wise there aren't alot of options in the Imperial large ship category due to the homogeneous nature of the Imperial Navy. FFG might release the lesser known Star Destroyer classes down the line but for now their priority seems to be ships from the TV series and films.
Also, I think FFG might be waiting to the see if TLJ introduces new ships (hopefully it does)before dipping their feet into the sequel trilogy.



