I agree that great generals shouldn't be just imported directly from BtS. FfH is so different that it needs some tweaking. I do not like the free xp thing - the great commander promotion is better imho. I think the best would be a GC promotion (granted as GG in BtS too) that allows to get a few special promotions, like strength increasing one or granting a few more xp. But not allowing faster xp gain, it is already fast enough in FfH.
It can be even just the same as current GC, just with a different method of generation
Also, the base counter must be increased - to at least twice the BtS required xp. And I believe it scales according to world size and game speed, just as normal great person generation. As for the xp for surviving a spell or withdraw - it is not so big anyway and withdraw grants xp in BtS too.
Regarding GG points from barbarian combat - I personally like the idea as I like to have many GG

, but with hordes of not so well organized barbarians in FfH it would be simple overpowering. I get most of the xp this way and GG should be something special, so better restrict it to wars with civs only. But I still think it will balance things a bit, as currently it is easier to get great commander as Elohim (especially as Einion) than as Bannor, Clan or Doviello.
As for the mechanism waiting to be used in Ice - I doubt it. It would be nice to have it used for anything. But I remeber there was a disscusion regarding generating heroes this way but it didn't make it (which I think is a good think). Well, but we can have hopes - The Team is full of suprises

I just hope if they do not plan to do something nice using combat xp, that we will get Generals instead.