TLDR: I don't think Neon will break the meta, and even if she does the reaction should not be to immediately cripple her. There will always be things too strong and incremental slow changes are almost always better. (except when a character is way too weak then oversized buffs followed by slow nerfs can really help).
New Releases: With the impending release of Neon, as typical with any new agent release there is a lot of speculation and panic about balancing issues. Most game designers will tell you that the best way to release anything new is to make it too strong (slightly) since perfect balance is impossible. If something is released too weak it dies quickly and fails to infuse the player base with excitement and interest whereas something that is too strong engages the player base and forms long lasting attachments which keep people playing after it is nerfed back to normal.
Neon: Her ability to run and slide is valuable and most importantly fun to play. Movement is the greatest way to make a character feel fun in virtually every game. I don't think it is broken competitively but we will have to wait for more details to know for sure. Her wall and moly look quite powerful and should be the primary focus of nerfs if needed. There isn't really enough detail about their stats but I don't see anything outrageously balanced. Her wall's similarity to phoenix is a potential issue, but it has much less flexibility, can't heal, and neon has no flashes which phoenix uses to combo really well with his wall. It is a piece for team executes which I like. Her ultimate is rather underwhelming to me. The trailer didn't show much but if it is just a beam of damage, the time to kill doesn't really impress me.
Balance Changes: SLOWLY. Time after time I see game developers (and the community) freak when something is even slightly too strong. Large sweeping nerfs that dramatically affect the playability of a character are so rarely effective at balancing said character. Valorant has done fairly well in that regard. When Sage was broken in the early days, there were a lot of complaints at how small the nerfs to her were. Valorant reacted a bit too hastily and kept nerfing her until she was pretty much gone from pro play. Small incremental changes with time to resettle the meta are key. Viper, jett and skye have received good changes that take them from outliers to good pickrates. Jett is still strong, and although the community is still clamoring for a Jett nerf, I think it would be a massive mistake to do anything but slow tiny changes. She is too strong but not in a game breaking way. Any serious nerfs come with a big risk of eradicating one of Valorants most fun and iconic agents from the scene. (I don't even like playing Jett).
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