Foil prices. Are they going up or down?

Every thing will go up in a few weeks, months, etc. Just give it time until the newbies that are selling their items for cheap will end up with 0 cards and the market price will go up.

Personally I'd sell now.

In my experience in the Steam Market, I have seen all items has decreased its initial value. If I was you, I sell it soon.

Once enough badges are made, prices will rise as supply drops

Everything's going down unless they implement another reason to craft badges, which is presumably what the mysterious cards are. Look at the price history for any foil, and you'll see a pretty steady drop. I'd sell ASAP, unless you're a speculator and you think Mysterious cards will reinvigorate crafting.

if i had it, i'd probably try selling it now

I'd sell. I have a HL2 foil. When i got it, i thought prices would rise. They dropped like a rock. Could have sold at around $7. Now the card is down to $3. Waiting for it to go up again, to what i could have initially sold it for. No one is going to pay $20 for a foil like some were in the beta.

Boosters also have foils in them. Some foils are down to $1.70.

I'm willing to hold on to it for a couple years even if it means that I could get a decent return. I'm not into Team Fortress too much but I imagine that some of the older rarer items are worth a lot more today than 2 years ago. Am I right? and if I am can I assume the same return will be there for foils?

It is risky either way. But what it comes down to is demand. Do you think people will want to buy a foil card for that game in a few weeks/months/years?... Also, you can see on the card's market page how well it has been selling (price vs how many sold), so that should help a bit.

Personally, i wouldn't think it would sell for that much later (or even now), not to be mean to that game, but it isn't like it is Portal 2 or some other major title. The people who want that badge(s) for that game are most likely getting them now, and i doubt that there will be an increase in demand later. But, as everyone has been saying, it all on you. This is your card, so you have to decide what to do with it.

There are already 5000+ of some individual cards on the marketplace. Even some foils are into the 90s, and that's with speculation on the mystery card holding back sales. So, the market is already flooded with way more cards than it will ever need. Unless Valve injects some sort of value to having cards with the mystery card, or via some other means, current market value is at its peak.

In other words, if Valve doesn't decide to alter the system again, the cards market is about to enter a DOTA 2-like wasteland.

You can never predict what will happen in the next few months or years. In my opinion you should sell now (also can getting $$ for summer sales). Even the price rise it won't rise much, at least not worth the time you spent to wait.

But just my opinion and I may wrong, as I said, no one can predict the future