Not really anything that can't be done on bitcoin or on top of bitcoin. Even some Monero advocates are excited about Lightning Bolt12 spec as it fixes some of the privacy issues lightning currently has.

The thing with altcoins are that, although if they have developed some interesting tech, it's not unique to that coin. If its good, and there are no trade-offs it can be implemented into bitcoin (for example, confidential transactions are live on Liquid, which is a bitcoin side chain), or on layers on top of bitcoin, which already exists such as coinjoins.

Bitcoin is uniquely optimised for decentralisation, security, and a predetermined monetary policy which is transparent. The rules are incredibly difficult, next to impossible to change without unanimous consensus without causing a chain split.

Altcoins doesn't have this robust, secure foundation and is often arbitrarily hard forked, and in other ways controlled by insiders by pre-mines, ICO:s, and owning infrastructure etc. So people who are building on altcoins are building on sand castles that has no viable long-term future.

There's also the question of anonset, bitcoin has a much larger anonset than monero.