Making it immaterial would destroy the balance of the card. Right now it goes
Pest.
Sanctuary! Ha!
Steal.
Oh.
And that's
exactly what's supposed to happen. Immaterial Sanctuary would be awful for game balance.
Like I said, add +1 to the cost both unupped and upped and you have a perfectly good card. If you decrease the healing and presumably the cost along with it, you're essentially relegating it to the realm of Purify -- a hard counter to a uncommon threat that has a minor side benefit to make it not a completely dead card. That's not good game design. Good game design is a slightly suboptimal card that you can still get good use from, that happens to include a hard counter to an uncommon threat.
Imagine how much more Purify would get used if it gave you 4 counters, removed poison, and cost 4: water or 3
upped. That's a respectable amount of healing even without the poison countering -- and that's exactly as it should be.
I don't understand at all the rationale that we should make the card narrower in it's focus. Keep it useful, increase the cost to match, and love it!