Cap'n Kobold
Hero
I'd be interested in seeing how well these rules work in countering the issue.
The potential problem that I see is for Clerics, Druids, Bards etc - classes that rely on spell slots to heal. The higher spell slots available do not compensate for the increased incoming damage, leading to having to burn relatively more of the classes' capability just to sustain the rest of the party rather on more fun stuff.
(Did you have a rule about being able to spend hit dice when you had a healing spell cast on you based on the level on the spell slot? If you're using that houserule as well, that might go a fair way to countering this issue.)
I wouldn't reduce cantrip damage however. - Outside of Warlocks, caster's at-will attacks aren't going to threaten martials'.
Remember they aren't taking away the ability to cast low-level spells. - Casters still have those available and even if they can't be upcast to match the slot, they can still be cast using the higher-level slot anyway.
The potential problem that I see is for Clerics, Druids, Bards etc - classes that rely on spell slots to heal. The higher spell slots available do not compensate for the increased incoming damage, leading to having to burn relatively more of the classes' capability just to sustain the rest of the party rather on more fun stuff.
(Did you have a rule about being able to spend hit dice when you had a healing spell cast on you based on the level on the spell slot? If you're using that houserule as well, that might go a fair way to countering this issue.)
I wouldn't reduce cantrip damage however. - Outside of Warlocks, caster's at-will attacks aren't going to threaten martials'.
I'm not actually seeing much of a drop in caster power compared to a more standard D&D game.Well, I strongly dislike taking away all the lower level spell slots from casters, as the continued relevance of many low level spells is something I like about 5e. But if it suits your goals, it suits your goals.
However, if you are going to take away people's lower level spell slots then you should drop the one spell swap-out per level-up limitation on memorized casters. You should perhaps also lower the cost of Wizards copying spells of the levels that will face semi-retirement since they are obviously of less long term value.
Or just do a campaign without any full casters, rather than worry about balancing a whole system of how to handicap them. I suspect any player who doesn't anticipate this being the permenent rules of all their future D&D games would just look at this scheme and decide to put off their next caster character for some other campaign.
Remember they aren't taking away the ability to cast low-level spells. - Casters still have those available and even if they can't be upcast to match the slot, they can still be cast using the higher-level slot anyway.