Re: another subscription level maybe?
The idea is original and perhaps it could be applied to the regular 4x4's and 5x5's, lets's say the easy puzzles (delaying the solution for a week, of course).
Being a subscriber has several advantages: access to extra puzzles (the puzzles themselves as a gift and 30 days for the solution), 2 more attempts for timed puzzles in 4x4's, 5x5's and 6x6's, some additional regular puzzles not accesible to non-subscribers, … . But having 7 days to solve larger puzzles or puzzles that actually require 3 days (12x12's on Thursdays and Sundays) or 2 days (9x9's on Tuesday, and Killer Sudoku's on Fridays) would represent an excessive advantage with respect to non-subscribers, in other words, it does not look very fair IMHO. All these puzzles are exceptions to the general rule, are more difficult and time consuming, and that's the reason why they have different conditions (particularly having two days for the 9x9's on Tuesdays is a very recent decision).
I understand that it is very frustrating that some specific days we may have trips, weddings, picnics, … , and we can not complete all puzzles before 24:00 hours CET, but this situation is common to all puzzlers. The philosophy of a site like this is scoring points within some determined period of time (in our case 24 hours), the philosophy of books is different ("infinite" time to score points), etc. In some way the extra puzzles (only for subscribers) are similar to book puzzles (you can do the last 30 days during a vacation, i.e., …
). But modifying the time limit for all puzzles would impact on several other aspects (difficulty level, … ). In the other hand, "curious" people, non-users, probably would like to see the solution soon, that is, the following day of the attempt, otherwise they would probably loose interest in the site.