I'm sure that guys like Ekmek and The Capo will master the challenge of creating LHs for civ5. I can't really understand the argument that most custom LHs for civ4 were only edits. Maybe those from the earlier days where, but that's also because plugins weren't working good enough then and of course because the modders needed some training. Look at the LHs from the last year and you'll see they are much better then the ones from firaxis and also pretty unique.
The problem will rather be, that they decreased the count of leaderheads to 1 per civ. I haven't bought civ5 yet and will wait with that for a while for the following reasons:
1) No time yet, due to reallife.
2) Most things that have been changed with civ5 were a big step towards the wrong directions IMO. Just like the leaderhead thing. Next, I can't resell my game if I don't like it, because it's AFAIK connected with the steam account. Sorry, but I have to state that I could understand if there would be a movement that uses 'black copies'...
Anyways, if it's true that leaderheads still give a boni, but there's only one per civ, then why connect that boni with the leaderheads? Instead, they could have connected the boni with the civ itself, making the leaderhead only an avatar. Well, seems like that's practically the case. Now, coming back to why that's a bad idea regarding leaderhead modding: the count of custom leaderheads will decrease. There is simply no need for all those leaderheads. Besides of some motivation aspects maybe as leaderheads became less important practically (due to the said changes). For a modder like me that simply adds some civs to the game, this will also be bad. It's sometimes hard to decide which is the best leader for a civ. I made a Hungary mod for civ4 and I added 6 leaders. That's pretty much, but hey: you can choose only one and at least you'll always find a leader that's traits fit to your desires. I just really can't decide if Árpád, Stephen I., Kossuth or Corvinus should be the one... And as player I wouldn't like that idea either. If my own country is available for civ, I'd want to mostly play with that. If I'm forced to certain boni because I can't switch between leaders and hence handycaped because those traits don't meet my basic startegy, I'd become pretty angry... Rubbish.
So this is the real downside of civ5 regarding leaderhead modding IMO.