Having the fireball only shoot forward is a design choice. I most likely also will not be adding a timer to the UI to show when the fireball is about to fire as well. It makes the game have a better risk/reward feeling, and incentivizes pattern awareness for skilled players.
As well in the future when more weapons/spells are added into the game,having a bunch of timers all over the screen would quickly clutter up the UI.
When other spells get added (which will be soon) I imagine the fireball direction will be ignored as you'll be firing off multiple projectiles all at the same time anyhow.