Complexity can come from many areas, and one area that rarely adds anything to any game is inconsistency. I'll be honest, I will probably be writing a new group of spells for my setting rather than do what I did last time and that was tack on around 600 spells to 3e to give the setting the feel I want. So I've written a lot of spells, and of the areas of the game its what I'm most familiar with.
For example, one of the things missing from 3e in these spells are descriptors. Those allow, at a glance, consistent and unambiguous rules for how spells of a given type interact. If a spell is has the fire descriptor, then there's no argument to be had as to whether it will be subject to resistances, or even function in an area of attuned anti-magic.
Before I go further on this spiel, read this
excellent article on game complexity by Mark Rosewater. While he's speaking on Magic: The Gathering specifically, the topics in principle apply to any game. He posits that there are three types of complexity, and here's how they relate to D&D - any edition...
1. Comprehension Complexity
What does the spell (or feat, or class feature) do? With D&D it's possible to have spells that conceptually are very simple to understand, but their execution grants a lot of latitude of interpretation (Pretty much all the illusion spells, and wish [ particulary the 1e version ] comes to mind). Spell descriptions need to be clear and concise. 4e does the best job here, but only by sacrificing most all the wonder and latitude of magic by reducing all the spells to "Do X damage, Slide Y squares."
2. Board Complexity
How does the spell interact with the others, and how do we deal with the options. Again, 4e does a good job here with it's over-simplified spells, but its best area was limiting of slots down to a more managable level. 5e has fewer slots, so this is a step in the right direction, but the removal of descriptors is a step back. 3e's descriptors allowed for more consistent rulings on how the spells interact with each other.
3. Strategic Complexity
Which spell is better? When? This sort of complexity Rosewater calls the 'safe' complexity since it is hidden from beginning players and as they discover it then in leads to wonderful "a ha" moments that help them feel good about themselves and their cleverness.
Anyway, it wouldn't hurt the game to be as consistent as possible. For example, how many area of effect shapes does the game really need? How many duration types?
Let's take that one. Durations are all over the map in 5e, as they where in 3e. But from a practical standpoint the game has these durations - instant, round, might last the whole encounter, encounter, work day, full day, effectively permanent.
Instant is easy enough - nothing to change here. Same with a single round effect.
Might last the encounter - that would be 5e's concentration. Spells in 3e with rounds per level durations a major culprit and tracking durations is a problem.
I've given thought to this alternate duration scheme for 5e. On each round roll a die dependent on your proficiency bonus 2 - d4, 3 - d6, 4 - d8, 5 - d10, 6 - d12. If the die comes up 1, the spell ends. This allows for a spell which is unreliable past the first round without asking the player to track a duration.
Encounter - that would be the 1 minute spells, or 1 minute / level spells. These things won't end during an encounter and tracking them adds overhead to the game.
Work Day - Any duration up to around 8 hours. The spell is going to last for this foray in the dungeon, but won't still be up that evening or during a rest. Fidgity durations like 10 minutes / level, 1 hour / level and so on fall here.
Full day spells can be any of the above in the hands of a high level caster. Effectively permanent spells are magic items unto themselves.
Clarifying this can't be bad for the game.
Casting times are just as bad really, even in 5e. The reality is there are 3 casting times in 5e - 1 bonus action, 1 action, can't be cast in combat. All the additional casting times are just flavor. Personally, I do like how 4e separated rituals from combat magic and would have liked to have seen that retained. In the last rendition of Dusk I had spells that could be cast for greater effect if they were cast slowly, as a ritual, usually by having a much longer duration. I don't know if I'll retain that moving forward.
Anyway, spell systems are something I love, and can be quite critical and analytic of.