Is there scope for having the javelin thrower as a technology for increasing infantry skirmisher range?

Both @Classic-Burger and @Deicide4u have some good ideas for singleplayer mode. If you want to playtest those ideas, I would recommend you to make a mod for yourself. That is much more efficient than raising suggestions on the forum that will very unlikely be adopted. If you encounter technical challenges, we are happy to help with specific coding difficulties and you can perhaps team up with @Emacz to make his Historical mod better.  Changing anything in the base game will go through heavy scrutiny, especially when tweaking something that can affect the balance, such as your additional techs. Most multiplayers use only the public mods, therefore we have to keep in pristine to prevent game-breaking balancing disasters. If you read the changelog on the gitea, you will see that most balancing changes are very minor increments in stats and something as major as your techs must be tested in many TGs before being implemented. If you don't want to you go through all this hassle to convince the MPs, just do a mod and submit it to mod.io. Hopefully some like-minded players would download it and 1v1 you there.    Back to the topic, I like how it's done in Delenda Est. The techs to take very long to research and some choice to accelerate it would be great. 

There should be chivalric civilizations with broken cavalry. Too broken but not that one class is above the rest. There will be tiers of units. We already know that missile units are support for infantry fights. But it's a shame that the forge technologies don't feel as powerful as they do in other games. RTS are fantasy games of power. Command powerful armies, feel the power of being a general and an urban planner. Armies should feel powerful by upgrading their weapons. There are no technologies for LoS, there are no technologies for repair... There are barely there are technologies for capturing buildings. The nerf thing happens not only in 0 AD, but in many competitive games.

I think you don't like the calls to "nerf" units , but that doesn't mean making the game more boring, to the contrary : what MPs players don't really like are too anchored 'metas' that make all games a bit too similar... Because one build or tactic is too hard to counter, and that therefor a players can repeat it regardless of the opponent being aware of what you he'll do. Ideally strategy, improvisation, adaptation would be rewarded over mechanically applying a cookie cutter build. You get a better experience from a game where you have to use your brain, teamwork, rather then just trying to be fast fast at spamming broken champ cav for example. Idk how exactly that translates for SP, probably just not being incentivize to make an army of 1 unit type, because it's stronger and isn't at risk to be countered, is already kinda giving the game an extra dept. Pretty sure that it's universal that players play RTS for the Strategy part (Even if I've been told that "Real Time" means it should be about being fast clicker or what not).

You’re arguing against a straw man. Very few people want to eliminate things the way you’re saying and most that do get ignored (for good reason).  The loudest the multiplayer community ever was was right after a24 got released and a lot of that was because features got eliminated. Note, when that happened a lot of SPs and devs initially dismissed the complaints before coming around later.  The most you hear now from the MP community now is that champ cav is OP, which no one has really found solution to. The other recent thing I’ve heard is reza saying fana is OP to which most people told him he was wrong. MPs will regularly say things like certain techs like “spies”are useless but even there it’s not like the SP is saying how great they are. 

Starcraft 1 is full of OP units, in fact. That might be why it became "balanced", every race had some nasty OP units.  I'd say Scythe Chariots and Chariot Archers were more OP than expensive Centurions. True, sadly.  Definitely agree here. You can always nerf the most blatantly OP units. That, and the fact they're naked. As in, without armor naked. 

I absolutely agree, we can be balanced without being symmetrical (as in aoe4). We have been through reduction eras in 0ad such as a24. We have come a long way in introducing varied gameplay mechanics and we need to continue doing that even if there are situations where things are imbalanced. For example fanas can be powerful at times, perhaps even op and perhaps in need of a nerf, but we should not revert their cost back to include metal as that is what differentiates the fana so much from other champs.