So, for now, my comments on LT43 that may be relevant for LLT.
1. This may sound strange, but I have a feeling Gunpowder actually came on too fast. It's not yet fully on, there are rumours that "a few" nations have it or may have it very soon. And as this go on, more an more will join in and things will accelerate. I'll make a wild guess that by T75 a significant number of people will have it. I think usually Gunpowder is discovered by T50 in normal LT games so this is a bit slower, but not much of a paradigm shift.
2. I think that full tech sharing compensates pretty much for bigger sci-box and that, generally, science progress is at a similar level with standard LT games. This may change as time progresses due to more expensive later techs, we'll see. Keep in mind that a more expensive tech means a bigger discount due to techleak
3. The room seems to be a bit too tight. This isn't necessarily bad, I've had (and am still having) fun suppressing our local Ghengis Khan with my allies, but it did throw us back a little bit (although, actually, not as much as I feared).
So, how does this experience serve ideas for LLT.
Tech definitely needs to be even slower than in LT43 (with caveat that we are yet to enter the Age of Expensive Tech; as it is now, the game will probably end by T200, but let's wait and see).
Also, I would be VERY MUCH in favour of "tech exchange by diplomats", so no sharing through diplomacy, but instead "stealing/transferring" it by envoys. It worked in LT40, although sometimes it depended on luck too much and all the parameters weren't completely clear. But I'd definitely go with trying to lift the "one steal per city" limitation.
The main issue may be that people won't have much to do in the first stages of the game. Also, long periods between scientific discoveries may be a bit anticlimactic. So, a few ideas regarding those.
I'd introduce some more techs and buildings. Maybe decrease the effect of the current ones so that Marketplace doesn't give you 50%, but you separate it into two improvements that give 25% each, or even better, separate the Luxury and Gold effect, each one having its building.
Of course, increasing the number of improvements shouldn't go too far because, once all impovements are built, we are left with building military units with not much else to do but go forth and use them, fun for all and no huge fallback if you get entangled in a war.
(A minor detail, but for a LLT game I'd definitely introduce another tech so that with Gunpowder you get "Hand Cannoneers" (Att: 2, Def: 2, HP: 20) and only after that a tech for Musketeers)
And now for the most important part.
With a very long game, people are going to get very bored if there isn't stuff to do. This is especially true for the first few dozen turns, while there is the primary expansion, cities are small, production is low, neighbours are very scarce and there is almost no conflict. And this will get only worse in a long and slow game.
So, my first thought about fixing it was: fill the land up with AIs so that everybody has something to bash without feeling guilty, creating bad blood or throwing other people out of the game. BUT, then I got a better idea: at the beginning, every player controls MORE nations that are actually delegated to him (or something). One is his primary, while all others have a dual purpose: for their neighbours they are cannon fodder so that there is more fun, while for the player controlling them, they are the means of annoyance and slowing down his opponent without actually destroying them.
Furthermore, this would add to the historical accuracy because the land through which ancient tribes and civilizations expanded were not empty: conquest and assimilation was a regular occurrence and I feel that there is far too little of that in LT. And, again, having people control more nations in the beginning would fix this.
Eventually, of course, a number of these auxiliary nations would vanish, Primary nations would expand their territory and things would become more compact.
But maybe this approach requires a test game? How about it? Four nations per player, 150 land tiles per nation, see where it gets us?