I've always said that sets in which a factory set was not produced are the best investment. Look at the values of cards in the 80's. Even though there were millions of cards printed, 81 - 86 Topps Football does really well. There have been indications that factory sets were made, but I have yet to see any. With new sets they are sold everywhere. Walmart has factory sets from the last couple of years on the shelf. At $30-$50 a pop, they are a hard sell because they are merely entry level sets. Sometimes there is a mantle refractor, a cheesy worthless GU card, or a 10 card prospect set.
Today the hobby has changed. Most every set that is worth collecting has short prints and there are enough insert sets to choke a horse. These are the sets that people try and build. Ginter, Heritage and goudey stand out. It is often very difficult to build these sets because of the number of short prints.
In other words, the sets you want you can't get, the sets you can get you don't want. So I was thinking of some ideas. Don't get me wrong, Topps probably sells millions of factory sets, but they become so worthless that I was able to buy 3 2004 sets for $5 each.
Master Sets: If Topps offered master sets of each year with inserts and limited the production to 2000 sets. I'd buy one
Autographs: In 1999 Topps put a hard signed auto in every Traded Set. Maybe try something like that again. It shouldn't be an exclusive to the factory set, these should be regular issue that were put into packs. The problem with exclusives is that they either do well or fail horribly. An example of good auto exclusive would be 2009 Turkey Red Inserts that are autod and only found in factory sets. IT's a strong checklist so it might be a bit too expensive.
Game Used or Relic Cards: This is trickier but could be done in addition to the Autos. If topps put 3 GU per factory set that are also found in packs, they would sell a ton because people would try to complete their sets by ripping open factory sets. Of course topps would need to put tamper proof seals on the box and not just the cello wrapping.
Non-Traditional sets: A limited run of Goudey, Ginter, Chrome, Heritage. Limited to 500 sets or so. Make them rare enough that demand would still be there for the wax.
Maybe most of you are just set builders are heart and don't need no stinking factory sets. Maybe vending boxes or packs that don't have inserts or autos or GU so the price is less?
Just some thoughts.