doctorbadwolf
Heretic of The Seventh Circle
Because shield proffeciencies on casters ussualy costs either feats or multi-classing (ussualy both or several feats since you almost always need warcaster aswell). The shield spell costs spell slots. A cantrip is free...Pretty big difference and obvious stuff, my dude.
OTOH, casters that don't get shields pretty much don't get them for flavor reasons, IMO. There's no reason to disallow casters to have good AC.
Still, most suggestions for this Cantrip are too much.
Not letting it stack with a shield makes sense. I'd also be fine with half proficiency, and take half damage from Magic Missile, if the goal is to actually just make the Shield Spell into a cantrip. If not, then I'd prefer something other than a flat bonus, like imposing a reroll, or giving an attack a 1d4 penalty, or something.Keep it +1 (maybe going to +2 when your proficiency goes to +4) but don't let it stack with a shield. You'll only use it where it actually makes a difference after all.
I would suggest against a scaling bonus to AC.
5e scales toughness with HPs, not AC. Part of the bounded accuracy paradigm. A +X to AC will stop about the same ratio of attacks, but stop a lot more damage at high levels. Scaling the AC makes it exponential - it turns more hits into misses times the increase damage for each miss.
5e doesn't scale spells by caster level (cantrips are a different matter) and that's part of the solve for quadratic wizards.
And as a side note, going by proficiency means that any dip gets it at full scaling. I don't suggest scaling, but if you do please make it by class level or caster level.
Well, that's just how cantrips work in 5e. If it does something where it would become useless to the caster at high levels without scaling, it scales by character level.
What I'd suggest for a spell that is supposed to be a minor version of Shield, rather than literally replace the Shield spell entirely, is either what I wrote above, or give a scaling THP and Resist Force until start of your next turn as a reaction. THP never stacks, so using it every time you're attacked can't cheese anything, and healing and damage are considered about equal by the design team, and THP is just proactive healing.
But what I want to know is, what is the problem being fixed? Is the Shield spell not doing it for OP? Then I encourage proficiency bonus (or half) to AC and Resist Force until start of next turn, as a reaction, and get rid of the Shield spell.
If they want a minor magical defense cantrip, I suggest any one of my suggestions above.
If they want it to feel like the mage is skillfully using magical force to stop attacks, I recommend a cantrip that imposes a penalty, specifically.