For a brief period in the design of original Zendikar traps were going to work like this.
Personally I don't think we are going to see this on instants. I think the installment aspect plays better than the warning of a trap aspect. Obviously they could do both but I think the warning aspect becomes less interesting if there are too many foretell cards in every color that aren't traps.

Another thing I just realized is that this is less interesting on anything too big. If something costs 6 and the only benefit of foretell is to make it cost less then you are left with a lot of words to make a very easy hoop. It's not hard to find 2 mana to spare before you draw your 5th land. The installment plan is really only interesting on things that cost 4 or 5.

Even if they're not traps there are a few situational cards you might not want to use right away but would be glad to pay for ahead of time. Board wipes, counterspells, oddly specific removal etc.
We might also see some interesting designs revolving around hand sizes or discard due to foretell.

When power creep has gone to far.

This looks balanced to me

This looks like a pretty cool mechanic actually. You can spend leftover mana to make a spell cheaper in a future turn or sacrifice curving out for a cheaper bomb, and maybe they have cards that get bonuses if foretold. Should make turns a lot streamlined.

Also BelangiaJo pretty much guessed it right.

I'm not really following along with he full spoiler or whatever this set is, but it seems to include some tribal components for some expensive creatures ; eg giants, dragons, and angels.

If one tried to build a tribal set based on these archetypes, one runs into the problem of flooding the high CMC part of the curve and then struggling to plausibly sell 2-3 cmc dragons and angels at the volume the set requires. If I was making such a set I would decide I need a mechanic that lets me push those bad boys out at lower parts of the curve.

In summary, the "interestingness" of such a design would not be in the inherent gameplay of the mechanic, but in the fact the mechanic enables a tribal set based on fat monstrosities.

Well they've also dealt with that by just introducing a lot of good lower CMC ones as well as like really cheap support cards like Magda and Starnheim Aspirant (Dragonspeaker Shaman for Angels if you didn't see it). But that is an interesting point