I fully support the implementation of more key modes.
I started rhythm gaming some months ago with osu!mania. I've played piano for over a decade, so playing 4k, 5k, 6k, and 7k wasn't an issue for me.
I was discouraged from playing 8k because of the lack of maps and the general lack of popularity. However, I'm now getting my 8k closer and closer to the level of my other keys. I think we should definitely allow for at least 9k and 10k support, as it brings me more of a feel that I'm actually playing the piano, which is amazing, especially with great hit sounding. Maybe it's also because I read chords well because piano, etc.
I think the osu!mania community should evolve to be more 'key-friendly' by letting each key be given the same level of legitimacy, maybe even 1k...
Osu!mania should be about playing any number of keys, not just 'another way to play IIDX or BMS or o2jam or stepmania, etc'
Not that we would be discouraging that, we are encouraging everybody to gain proficiency in all key levels because it creates a more even and balanced game for everyone to enjoy.
Mappers like Blocko and LordRaika are doing a great job in this regard, by giving a full set of difficulties for every key (currently rankable), and I would love to continue in this direction.
TYB mania development.