I think this is a fantastic idea! In fact, if I’m allowed to toot my own horn a bit, this is something that I came up with myself quite some time ago. I’m very pleased to see that WotC designers came up with something so close to what I use in my own game.
Previously, as the caster’s level went up, the spell effects of many spells increased accordingly. In 5e, all spells (with the exception of certain cantrips) now have the same effect regardless of the caster’s level. However, the caster has the option of casting some spells at a higher level by using a higher level spell slot.
I’m not privy to the thought process behind this, but from the way it is presented it appears that they are attempting to tie the effectiveness of spell casting to the resources (in spell slots) used to cast each spell.
I am totally behind this. I believe it is very reasonable and a welcome addition.
However, I came to this idea from a different direction. I’ve always had saving throws against spell effects influenced by the level of the spell. If you cast a low level spell against a high level opponent, that opponent has a very high likelihood of making the save and resisting (at least in part) the effects of the spell. If you want a greater chance of affecting your opponent, you need to cast a higher level spell. The obvious problem is that there isn’t always a higher level spell with the same effect. You could, of course, create more spells to fill those gaps. But a more straightforward approach is to be able to cast spells at a higher level.
In my game, wizards use spell energy to cast spells. In order to cast a spell at a higher level, they simply expend more energy. In this way, any spell can be cast at any level, up to the level of the caster. Furthermore, a spell can be cast at a lower level. For instance, if you want to impress a group of primitive tribesmen with a flashy fireball, but don’t actually want to burn their village to cinders, you could cast a low level version that does little or no damage but still looks amazing. It may sound like there would be very limited applications for casting a spell at lower levels, but in practice my players have actually discovered a great variety of uses. The one restriction is that you can’t cast a lower level version of a spell until you are able to cast the spell at its normal level first (and of course locate and learn the spell in question).
However implemented, I think the basic idea is fantastic and makes spell casting much more interesting.