That's actually easy to do in the current system. What you need is a slight tweak that allows for cantrips to be cast using higher level spell slots (like other spell levels).Again, in general, I don't disagree. I would potentially provide a greater range of at-will abilities (or ways to augment them) and less of the high power shenanigans. A lot of answers to @Morrus's hypothetical depends on a lot of unknowns that we aren't privy to, such as whether wizards and/or other spellcasters would also be in this sci-fi D&D game.
You could potentially even have a class that was nothing but a selection of cantrips and a whole lot of spell slots they could spend only to upgrade those cantrips in particular ways and it would be an elegant retweak of the existing system. (Although I expect that would be going to far - at the very least you'd probably want to include some kind of Warlock once a day feature for casting spells level 6 and above - and I'd make new previously unavailable cantrips available at the levels where other classes normally upgrade their cantrips).