Scalability always being a problem on cryptocurrency, example like Bitcoin everyone always debate about that. But atleast we already have some solution for that Segwit & Lightning Network. To be honest for future cryptocurrency always make some new progress about their development.
I would have to add cross chain interoperabilities along with scalability. These are two big topics that are in coversation right now. There's a handful of platforms looking to acheive these things. I know of Moac as a prime example but a couple others should be launching later this year.