from what I gathered,
all mechanisms are done save playtesting and bug chasing and balancing..
but more cool stuff may be added before ice, without adding any new major mechanism.
for me that may mean : new units/tech or new civics or new events or new equipements or quests, leaders, builldings, wonders...Etc. (especially the ones that won the design polls..)
and that also means that normally, before releasing ICE, the FfH game will be completed (all stuff enounced before). ice being only scenarii to complete the lore (and maybe add some cool units and equipements)