For Franklin, compare Isaac Newton: He gives a max of 3 Sci rating in Age II, 5 in Age III, and no CAs needed to discover techs. Now, no CAs to discover techs is nice, but you don't have the extra 3-6 Sci per turn to get them, which means a lower return. That said, I think he needs more than just the tech discount--perhaps 1 Culture instead of 2 Sci?
I don't think the "costs less to play techs" is a good change though; a couple of times, I was limited by not having the Sci to pay in advance. I think that change would make him more powerful.
Newton can never give more than 3 Sci, because he gives extra science equal to the age of your best lab/library. There's also the matter of having to work for the bonus, he gives you more of something you're good at, not just more science. Costing less is actually only a minor boost compared to getting it after, it's more for neatness. And well everyone else is getting the same thing, 1 less to play techs. I think that on it's own, without any science rating boost, would be balanced.
Croesus gave me a huge, huge boost at the start of the game; I think he was OP as well. Consider that I passed on all the Age I leaders because of the ability to rapidly play yellow cards. "Once per turn" would probably help with that.
Yeah, I definitely underestimated Croesus. He basically adds an extra CA onto every action, which means you can spam action cards to help you build up easily. Looking back, you played 6 action cards in Age I, which lasted 6 turns. So I actually wonder if once per turn would be penalty enough. I wonder if he might work better as, say, you get 1 CA per turn, which can only be used for taking action cards (as opposed to using them).
I would have loved to see the Mausoleum finished and used; Croesus, Nebuchadnezzar, Pliny, or Darius could all have been game-changers even in Age III.
Lighthouse seemed OK; I'm not sure if "only applies when defending" should be upgraded to "does not apply when attacking."
It got to near the end of Age I and Galz was saying how he couldn't finish his Wonder, which was something that wasn't quite true I believe, although it would have been tough/required some luck. It definitely had some interesting combos.
The Lighthouse, I can see the argument for that. It would be very similar to Biskupin at that point - giving 1 more strength and being easier to build, but not giving a yellow token.
I agree that Reality TV was underpowered. Probably so was Human Genome, Cotton Gin (compare Transcontinental Railroad), Gutenberg Bible (only applying to libraries makes it tough), Macbeth (pacts are... not very exciting), Goethe (sure, 6 culture, if you can get two of Library, Arena, Theater, and build them), Elizabeth (I made her thematic (think destroying the Armada) but not very powerful.
HGP I think is fine. Perhaps too expensive, but Labs are cheap, and Farms are cheaper AND have good combos (impacts wise, notably, but also Borlaug) AND aren't limited by your building limit.
Cotton Gin I agree. I think you could probably throw 2 culture onto that, change it the best farm, and it's still be pretty situational.
Gutenberg Bible, I had the same thought about. I think it's actually an okayish wonder if it comes out at the right time - when there's a good chance you'll miss Alchemy in particular. It makes going for Journalism look very attractive in age II. It also combos well with construction techs. 5 cost Journalisms? But really, I think probably 1 less resource per age would be better.
MacBeth I think is fine. The big thing with him I think isn't the pact stuff, it's 1 MA at a time when 1 MA is really nice. The pact bonus is just gravy, especially if you manage to draw Trade Routes.
Goethe I also think is fine. He compares favourably to Shakespeare - yes, Shakespeare isn't great, but he's an Age II culture focused leader, and I think that he's fine for what he does. 6 culture per turn is nothing to sneeze at. 12 culture per turn is alarming, even if somewhat difficult to achieve.
Elizabeth, I realised the fun thing with how she's worded means you can sacrifice your Cannons for +6 strength, which is quite cool. I do agree she's not that great though. I might suggest changing her to "Your artillery units have +1 strength. When defending, your artillery units instead have +3 strength". That +1 strength would then apply always - during strength comparisons, and when colonising.
I think Impact of Happiness is capped so you can look at your Happiness Track. Also I think Happiness is so cheap to raise endgame compared to other things that if you knew it was coming, you'd be at an unfair advantage. Like, Engineering Team Sports - 5 resources for 8 culture, up to 4 times? Throw in St. P's Basilica and you have something absurd. No Impact can (I think) come anywhere close to that level of increases.