*Disclaimer*
I didn't play the playtest yet, so if i am horribly wrong somehow, just tell me.
When reading about spells, one thing seems to be totally obvious to me: You will never prepare a damage spell that is significantly lower than your level, because that damage spell will surely be eclipsed by any of your damaging cantrips.
I understand the reasoning for limiting heightening of spells and i am agnostic to that... maybe it was really needed. But i think some kind of change is needed when every single damage spell is useless after a few levels if it's not heightened. And since high level spell slots are a very sparse resource, most probably wont.
I have an idea to make those low level spells at least a bit usefull. How about a feat that allows us to cast a spell at least 2 spell levels below your max for 1 action less? That way you could do a cantrip in the same round if you want. It would reflect the fact that such low level spells have become easier for you to cast. And low level damage spells would still be somewhat usefull, even when not heightened.