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I think an interresting thing to do with daemons would be to make their invulnerable saves dependant on the AP value of the weapon which wounds them.

Eg.

AP 1/2 weapons would allow a 6++ (5++ for greater daemons and tzeench daemons)
AP 3/4 weapons would allow a 5++ (4++ for greater daemons and tzeench daemons)
AP 5/6/- weapons would allow a 4++ (3++ for greater daemons and tzeench daemons)

Do you think it's a good idea or just ridiculous?

Too much bookkeeping, IMO. Why do you think this needs to be done? In my experience, Daemon's survivability is fine as is.

So, you'd want to make their invulnerable save # go up based on the AP value of the weapon used against them? That strikes me as defeating the purpose of giving them all invulnerable saves to begin with as opposed to armor of various kinds.

Might not be a bad houserule to try out for armor saves themselves though, the 'all or nothing' approach could do with a graded varient. (for an codex that has them, that is).

So make daemons even worse? No.....

It would make more sense to give increase invul save as if they were going to ground (i.e. if they don't move improve save by one; there are immaterial beings after all). Most times the daemon units want to close in; trade off mobility for survival. It's simply, easy to remember and gives the daemon players some flexibility and tough (not auto) choices.