I still like the general concept of representing English industrialization with an early steam mill, but need to add some effect to it that will significantly change English gameplay.
This is why I like the frontier fort. +50% growth rate is huge and can let newly-settled cities catch up to the others very quickly, while simultaneously protecting them from attack. It has the potential to fundamentally change the way Washington plays throughout the entire game.
Your variation on Rhys' idea - gold from every specialist - is a good one, in that it's powerful, and that it should steer England in a certain direction. Given their other traits, it's not OP, either.
I've always really liked the Frontier Fort, and if we can't find an equivalent that works in a later era, would be very happy with that.
The German barb notion is a fat pitch that has been waiting six months for somebody to knock it out of the park.
I couldn't be more in favor of making the Leaders more distinct. I would keep in mind that it's a small and inexact poll; Arabia was my #6, and I play with Songhai and Spai some of the time. More to the point, some of the civs you mentioned changing strike me as among the most distinct of the bunch. Unless a civ is also UP, I wouldn't change any of the DLC civs, which were all clearly designed to stand out. Specifically:
It's generally agreed that Babylon is a top-echelon civ, with as specific an optimal path as any. They don't need a buff, or to be made more distinct. That seems to seriously narrow the reasons to change them.
The Mongols are also super-distinct, and I don't think anyone finds them UP. Your suggested change narrows their path, and makes them less devastating. In this case I think you might be forcing the optimal way to play a little too much (and similar to Songhai). But I'm not sure.
Spain is a boom-or-bust exploration civ. I would imagine that this goes against your preferred playing style, but it doesn't go against everyone's. Sometimes I am in just the mood for a roll of the dice where I wind up with either a borderline-OP situation, or with just a decent civ. Like all of the DLC, Spain is very distinct.* If that's what you're going for in general, then I'd consider leaving them alone as well.
However, there might be a sweet spot satisfactory to all that still makes exploration the way to go for Spain. This would be a Natural Wonder buff that pays less for being first, but more for being an also-ran, and that also buffs the bonus for settling near one.
Moving on to the others, your proposed buff to Rome wouldn't make them much more distinct, but it would make them better. It's a good idea, pushing them up to strong-power status. (A lot of what you're doing also raises many civ's games, which is al to the good.) Another way to go here is to drop the Ballista and buff their Workers.
The Songhai happen to strike me as pretty distinct, and definitely not UP. The science-temple proposal doesn't make them more distinct or more fun to play - just a little better. I'd view this as an okay but relatively minor change.