I suppose it would depend on the original range of the spell. However, note that many spells do a set amount of damage based on spell level, so damage caps may have to be reduced.
For instance, if we modify Fireball to be a 2nd level spell, we would change the range to self (but remain a 20-ft spread), allow immunity to the caster, and lower the damage cap to 5d6.
Im not sure it would work as an inverted metamagic as those things have a habit of becoming too powerful.
For Horrid Wilting I would advise setting some sort of base to what the spell can affect. For instance, the "Target" entry could read "Living Creatures, none of which may be more than 60 ft from the caster". This is essentially what the spell is, except the spell allows the first target to be at Long Range. I would allow a max of 20d8 (5 less than the original spell).
I think that would make for a comprable 7th level damage spell (compare to Delayed Fireball).
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