Adoption is where my mind really struggles.
Well, I think a pretty significant part of this is luck to be honest. I mean, you cannot have adoption from the start which means the said coin could end up with a quite high level of adoption, could struggle with it or the devs & community could simply give up after a while - basically killing the coin.
As you said, there should be something
new in the space. Something like what Mimblewimble came up with - and even mw kinda struggles.
If the Bitcoin markets seem like they're starting to mature a very little bit, altcoins are still very immature at this point. I'd bet anything that once the Bitcoin bull run begins, no alt (I'm talking about those with high trading volume and liquidity) will surpass it percent-wise. People move from trend to trend and tend to forget about the purposes of various coins.
Once a new idea pops up, it's kinda hard to be among the first to find it and, at the same time, it's quite hard to know whether people will stick to it or leave it. Luck is part of the game and you have to accept it.
I have personally tested lots of alt investments in various niches but found out that the
only coins that I am interested in and using properly are privacy-oriented ones. All the other coins serving so many other purposes are useless to me, so I've backed off and focused on privacy-oriented cryptocurrencies instead. And even with these, it's hard to find ones that actually bring something new and useful in the game - and even the new stuff may sometimes come with the compromise of security, which is
vital for coins that are supposedly protecting your real identity.