So while everyone was arguing over how spell crits should/shouldn't be removed I was thinking of how to change them instead of removing them entirely. What I came up with is instead of having them deal double damage, they can have a unique effect based on the spells damage type

Fire: Target takes 1d4 fire damage at the start of their turn for a number of rounds equal to your proficiency bonus.

Cold: Targets movement speed is reduced by 10ft for a number of rounds equal to your proficiency bonus.

Force: Target is knocked prone.

Thunder: Target is deafened for a number of rounds equal to your proficiency bonus.

Poison: Target has disadvantage on constitution saving throws for a number of rounds equal to your proficiency bonus.

Acid: Creatures within 5ft of the target take 1d4 acid damage.

Necrotic:?

Radiant:?

Lightning: Target has disadvantage on attack rolls for a number of rounds equal to your proficiency bonus.

Psychic: Target has disadvantage on wisdom saving throws for a number of rounds equal to your proficiency bonus.

Now bear in mind this is made quickly and with little thought but I think with some polish it's a good house rule.

Sounds like this would make good repeatable feat. I would add:

Necrotic: Target can't heal for a number of rounds equal to your proficiency bonus.

Radiant: Target is blinded until the end of your next turn.

Quote from Conren >>

Sounds like this would make good repeatable feat. I would add:

Necrotic: Target can't heal for a number of rounds equal to your proficiency bonus.

Radiant: Target is blinded until the end of your next turn.

 I could definitely see it as a feat in 5e but since they've removed spells being able to crit in 5.5e I figured this would make a great alterative so that way casters can still look forward to criting on attack rolls.

A house rule for playtest material?  Wait until the final procedure gets made.

As for using this idea to make feats?  Some of those are overpowered.   Typically I would restrict things to one round or make the feat useable once per short rest on one target.  Keep in mind that people could use spells to attack EVERY round and some times a single spell could be used to make multiple attacks.

Fireball burning 5 victims for multiple rounds, spiritual hammer knocking a new guy prone every round.  etc. etc.

I agree that the multi-round effects are probably a little too good... on top of that, it adds a lot more bookkeeping for the DM, who needs to keep track of which creature got hit, what damage type it was, and how many rounds it has been, as well as what the player's proficiency bonus is.

I don't think there's much worry for something like Fireball... this concept seems to be built for spells that require an attack roll, and I'm not sure if there are any spells that call for an attack roll where more than one creature can be targeted by a single attack roll.

Quote from TransmorpherDDS >>

I agree that the multi-round effects are probably a little too good... on top of that, it adds a lot more bookkeeping for the DM, who needs to keep track of which creature got hit, what damage type it was, and how many rounds it has been, as well as what the player's proficiency bonus is.

I don't think there's much worry for something like Fireball... this concept seems to be built for spells that require an attack roll, and I'm not sure if there are any spells that call for an attack roll where more than one creature can be targeted by a single attack roll.

 Eldritch blast is what I think of but aside from that I cant think of any off the top of my head. And like I said it definitely was not balanced/polished. But I feel like its alright for a first draft.

Eldritch Blast could potentially hit multiple enemies, but it's still a separate die-roll for each attack. So the odds of critting against multiple targets in a single turn are pretty low.