It's quite good idea and can be enjoyable for a while. The graphics are good and fluent, the moving coloured lines and the combo support the fast-paced feel very well – but if you are quite confident in your ability to spell out the alphabet in order (if not, the letter you should pick is even shown at the top), it gets repetitive. Aiming for high score, levelling up and maintaining the highest possible combo keeps you entertained, but not indefinitely. You can't even pick a wrong letter – there's no penalty either, aside from wasting time. Why not add some variety or different game modes (unlocked as you level up)?
Just as an example:
– Don't show every letter of the alphabet consecutively. Start the game with handful of letters and add new ones randomly. Player will still select "the lowest" letter, but now it requires a bit of thinking.
E.g.: KGDZ → (D selected) → KGZ[U added] → (G) → KZU[A] → (A) → KZU[S] → …
In this case, you'd probably have to prolong the combo timer for a bit.
– Selecting wrong letter results in lowering the base score of all currently displayed letters by 2
– Negative value letters you are supposed to ignore (max 1-2 per screen, will disappear in time)
E.g.: "A -3" – if you pick it, -3*Combo will get subtracted from your score; you should instead pick "B 6". "A -3" disappears as you do.
– "Scrabble mode" - form a word from letters displayed (although this one is definitely easier to say than to implement)
Also, the fullscreen mode Unity adds in WebGL export is quite a mess here, but since fullscreen mode isn't needed for this game, I wouldn't bother with fixing the layout on different resolutions. Instead, I believe you can choose "Minimal" template for the export which doesn't add the "Unity WebGL" bottom panel with the fullscreen switch (nor there is a loading bar).