I think having a sandbox on the main site might be worth trying — not so much because of the 5 rep threshold, which I feel is pretty much a non-issue for the reasons SztupY describes, but simply because it might increase the visibility and use of the sandbox, and thereby improve the quality of the challenges.
We should at least give it a try — if it doesn't work out, we can always migrate the sandbox(es) back to meta.
Regarding some issues raised in the comments above:
We should probably close retired sandboxes on the main site. This would prevent them from being bumped by new proposals, and should thus cause them to gradually sink off the front page. (Comments don't bump threads. Answer edits do, but presumably those would taper off gradually after closing, as the old proposals are turned into challenges or just abandoned. I might expect to see one or two sandbox questions on the front page, but rarely more than that.)
The boilerplate text for the sandbox questions could suggest sorting answers by activity. In fact, it probably should do that anyway.
If people really want to ignore old sandboxes, we could consider making a special tag like sandbox-closed for them that could be ignored without ignoring the current sandbox. Anyway, note that even ignoring all sandboxes doesn't actually hide them — it just grays them out so that they're easier to skim over.
The fact that deleted answers can clutter up the answer list is annoying (to 2k+ users), but not specific to sandboxes or to this site — it's annoying on any question that has a lot of deleted answers. The best solution (short of convincing the SE devs to show deleted answers in collapsed form by default) is probably a user script — if there isn't a suitable one out there already, I could try to write one.