Haunt is one of the most underused mechanic IMO. And with some tweaks it could be a major mechanic in a set.

It could be. I wonder why they didn't make more of it.

Ah. It's because the second part of haunt is not optional.

The haunt mechanic is actually:
"When ~ goes to the graveyard, exile it haunting target. If gains, when ~ dies, {whatever}"

So yeah. Haunt is basically useless. This is a much better version. (It still has difficulties mind you - it's fiddly, it leaves your cards under your opponents cards, you have to let something of yours die)

• "The haunt mechanic is actually: 'When ~ goes to the graveyard, exile it haunting target. If gains, when ~ dies, {whatever}'"

This is incorrect. The second part is entirely optional, which is why it's written as rules text and not reminder text. As long as you use it only on permanents and if you tweak the rules to allow a parameter (the same way hideaway and echo were tweaked) there is no issue.

In fact, the example of hideaway shows that WotC actually would be willing to tweak a mechanic that has a nonoptional mechanical part by retro-actively making it optional (ETBT in hideaway's case). It just so happens that the part is already optional in haunt's case.

The changes to make this work as desired are all within the ballpark of things we've seen. And there are a lot of custom card designers that have desired them. It's just a matter of WotC figuring they want to do the same thing for a set.

It's everyone's guess as to whether that's a "when" or an "if". But MaRo has stated again and again that the biggest issue they have identified with the mechanic is that it works differently on permanent cards and nonpermanent cards - I say usingit only on permanent cards henceforth (or even for a given set) solves that issue already.

Allowing a parameter in this way is something I had considered for a few keyword abilities (including haunt, in the same way you have done; but also mutate).

Haunt only means "When this permanent dies, exile it haunting target creature." (the nonstandard parameter allows it to target something else, such as a land in the case of this card). It is now "haunting" that creature (or other object). It does not mean "If gains, when ~ dies, {whatever}"; the rules say no such thing. The card that has haunt might have an ability that cares when the haunted creature dies, or that might affect it in other ways, but it does not mean such a thing by itself.