Single-use Techs tend to feel either too strong or too weak--some replacement concepts:

Whether or not you feel they "fit the game," it's not controversial to say that DE's insta-techs (brought into focus by the LotW expansion) are unbalanced. They're either too strong or basically useless. Personally, I also think they eat up some of the civ's design space for more interesting techs. Here are some concepts for the techs to get some of the same flavor but in more permanent ways (and with more options to tune with).

Paper Money (Vietnamese): This is still underrated by the community it seems, even now that it no longer costs gold to research; it's a great boost for 1v1s and very valuable in team games.

• Proposed Change: Lumberjacks generate a trickle of gold, similar to the Burgundian Vineyards tech for farmers. It feels more thematically appropriate with the tech name and is a nice boost to the Viet eco in late-game. You can tune it to anywhere between 30-40 lumberjacks = 1 relic income and it would give their long-term income a nice boost without it being an overwhelming advantage. The tech will need to cost gold again for the initial investment.

Cuman Mercenaries (Cumans): You never research this, and if you do your allies never make use of it. If they do, they'll lose the 10 units pretty much immediately in whatever 40+ unit battle is happening in Imperial. The fact that it takes Castle time to train the Kipchaks is good in theory, but practically makes it worthless since castle time is important (especially for civs that would actually make use of Kipchak). One of the weirdest techs in the game, and it doesn't add anything to a civ that lacks a lot of options in the late-game.

• Proposed Change: Cavalry Archer cost changed to 60 wood, 20 gold. This would give the Cumans access to a powerful pseudo-trash unit that lacks Bracer. It's worse than their Kipchak but the faster training time from the castle age UT (that usually never sees any use for the CA line) and cheaper gold cost would make for some interesting options in post-Imperial. A similar idea to the Poles cheap knight spam but relegated to Imperial, and in keeping with a cheap, mercenary vibe.

Flemish Revolution (Burgundians): The potential to receive 200 instant army should have never been in the game, no matter how much they've nerfed the cost and the stats. It's hard to resist the temptation to abuse it, particularly on maps like Arena (I am definitely guilty of this). This single tech makes Burgundians the #1 civ on Arena is well-hated among the top players. No one feels good after winning with this tech, and it certainly doesn't feel good to lose to it. It's not unbeatable, of course, especially if you know it's coming. But if you don't have enough time or if you don't have the right civ, you're out of luck. Once the tech's been clicked the game is over either way. To it's credit, it does have a unique feel that is hard to replicate.

• Proposed Change: The easiest (though admittedly boring) suggestion is simply remove the villager conversion aspect. Make the cost much cheaper and have it enable the training of Flemish Militia the TC. A halberdier/champion combo unit is strong enough on it's own. This way it competes with villager creation time and spawns from buildings that are usually far from the front lines. You could mass it to use offensively but it would be expensive, slow, and awkward. Why use it? Because it's a powerful, cheap unit to defend your town with when it's getting raided. Make it very fast to train (like Obuch speed) and your TCs/villagers are a lot harder to harass with Hussars. Or nerf the unit itself, make it a Karambit-style glass cannon that is very cheap and costs half-pop. There are lots of fun ways that a flood of quickly-trained militia men defending their home could be in the game.

First Crusade (Sicilians): Another odd-ball to rethink. Personally I always felt that this should be tied to Donjon count instead of TCs, to reward aggression rather than booming, but the instant spawning and above-pop cap potential has been proven to be very, very strong. If the idea is to go for a flood of units in that First Crusadery way, I thought of something odd but fun.

• Proposed Change: Keep the conversion resistance increase. Enable to training of other units at the Donjon in addition to Serjeants. This would give the Donjon the most unique building identity in the game--not only would it be a tower/UU production building, but would have cross-utility with other production buildings. The more units you give to the Donjon, the stronger it would be. Honestly, even one other unit to train would be very, very strong. Halberdier is the most obvious choice, allowing you to mix in anti-cav with your strong infantry front-line more easily. Other good options to have instead of the halberdier would be other trash units (skirm or light cav) and/or rams. This keeps the identity of the Crusading unit flood but ties it to their aggressive, expensive unique building. Their snowball potential does increase, but we've rarely seen the out of control Donjon pushes we all feared when this civ first dropped--the towers are surprisingly weak. This would buff them with more versatility and reward aggression from an otherwise very boomy civ.

Final Thoughts: These are ultimately fun concepts I wanted to share with the community. They're here on their own merit, separate from other balancing needs these civs might have. I feel like there's potential for a lot of fun uses for various techs/bonuses (Incas team-game bonus rework please, +3 LOS for herdables let's go) that are currently in the game but aren't fully living up to their potential. The latest expansion was full of fun, new ideas that are a blast to play with. Let's see these problem child techs have a new life!