EzekielRaiden
Follower of the Way
I had had some ideas for a truly simple caster as well. My main concept was to, more or less, have it built around an upgraded-to-full-spell prestidigitation of yesteryear. That is, a core magic "trick" you can do a certain number of times that can be applied in particular directions. Relatively constrained compared to the bag of tricks a Wizard can have, but still quite diverse compared to a Champion Fighter. (I don't expect a "truly simple caster" to be as simple as the simplest possible martial character, to be clear, but it's gotta be much closer than any current caster.)Yep, I agree with all of this.
For a Simple-Caster, I'm envisioning something like the Warlock, just minus the spells and most of the invocation choices, but with more options for warrior-damage scaling than just Eldritch Blast. Some extra resilience and mitigation abilities, and something like Action Surge (maybe "Overload Cantrip", that lets the cantrip hit an area once or twice a short rest.) Tack on bonus action short-range teleport or some other defensive/utility boost, and put a bunch of flavorful utility options into the subclasses.
Complex martial could go a few different directions, but some combination of warlord and paladin (battlefield controller and party buffer/protector) or a ranger/rogue/blood hunter (lore and skill expert, has a trick or trap for every situation) would be my initial thoughts.
The Warlock comparison I think is important but not quite perfect. Warlock's shtick in 5e is the "build your own class", to a degree even more pronounced than Bard. So, more or less, I'd probably interpret the "simple caster" (I called it "Mage") as having Bard-like subclasses--where each subclass sets you up for some pretty specific focus--but with expression more like what a Warlock is like when it's gotten all the invocations for a particular path. So the "Valor Bard"/"Blade Warlock" equivalent would still have that core magical action thing, but would also get a few specific ways to kick butt with weapons too (EA at 6 is traditional, for example). The "Lore Bard"/"Tome Warlock" equivalent would instead get some supportive and ritual-leaning stuff; perhaps Mages don't do rituals, but the Ritualist subclass gets access to a package of ritual-like effects at ritual-like cast times and durations. Etc.