From: The True North. Strong and Free
I'd be more optimistic if 2by3 was making the game engine to be honest. It sounds as if Slitherine are making an engine one size fits all kind of thing.
Still I want top down graphics, WEGO\ Simulation turns (this should be the major new feature for SP\Tactical wargame made by 2by3, I also want squad size units and individual vehicles. Campaigns, dynamic weather, detailed damage model, multi story buildings, Cellars, Fortified buildings, sewers, be able to breach walls, leaders, detailed AAR's, award\medal mechanic, detailed unit stats, huge selection of vehicles and weapons, a good close combat mechanic, air support, off map Arty, Counter Battery, well made maps with decent amount of terrain features like walls both high and low, raised railway embankments, ravines, variety of crops and different heights depending on season etc etc, maybe get rid of hexes and use pixel movement this would help solve oddities with regards to terrain abstraction , high level command for very big scenarios so you can give orders to platoon or even company, good selection of SOP's, a good choice of FOW settings\options upto friendly FOW, AI Allies and finally a great TAC AI.