Here is a small list of suggestions that I have come up with for your future games.
1. Secondary quests - most adventure games suffer from a lack of replaybility, so adding in some optional quests would be nice. Like the ones in SBCG4AP, where you had to gather manual pages and raise your coolness level. But instead of finding the pages under conviniently placed boxes, have the player actually have to solve a puzzle or two to earn them. This brings me to point...
2. A scoring system of some sort - surely you remember old Sierra games with their scoring system. Well, why not do that, but with a modern twist? Like take into account the time it took the player to solve X puzzle (the bigger ones of the game), how many secondary objects he has found (like the coolness level in StrongBad), how many times has the player used an object incorrectly, etc. Then have those statistics give an overall score and rating (like in Resident Evil games).
3. Achievements - sure, the Xbox 360 versions have them, but what about PC? We want something to unlock too. And it shouldn't be something really simple, like solve a puzzle that's part of the story anyway, but for doing some additional stuff (like an achievement for doing all pranks on all of the characters in StrongBad, which was not essential to the development of the story, but it's a nice and fun distraction from the story puzzles).
4. Harder games - I'm sure that this has already been discussed, but Telltale should really make the games harder. There's already a hint system in the games, so new players will likely not get stuck, while veterans will get a better satisfaction out of playing them, so it's a win-win situation. Let's face it: nobody likes to complete a game in 2 hours, even if it's a 9 $ one.
5. More environments and objects - this is really needed if Telltale wants to make the games harder because there are only so many combinations that you can make with 5 objects in 5 locations. I was quite surprised that some of the games didn't even have the inventory reach the border of the screen.
6. ALTERNATE PUZZLE SOLUTIONS - LucasArts experimented with this in one of their Indiana Jones adventures, but Telltale has the power to really push this concept forward. I know that it might be a pain from a design point, but you guys should really give it a try. Start off with one or two puzzles per episode that can be completed in different ways, then work your way to more and more. This would be a huge bonus for replayability, PLUS an added feeling that the player can actually control what is going on, instead of just being guided in a single direction that the producers have thought of. This would actually go very well with point 1) on this list. If a player completes all (or some) secondary quests, he may be given a special object that allows him to complete a puzzle in a different way. The ultimate story would still play the same, but it adds a little flexibility to the mix. And it doesn't have to be for ALL puzzles, just some of them, maybe the bigger and more complex ones. Let the player get creative a little bit more.
7. Item combining - this is pretty obvious and I strongly recommend implementing it in all future titles. It's a huge component of any true adventure game and also has an important part in the difficulty of the game. But here's where this ties in with point 6)! What if you had an item A that you could combine either with item B, or with item C? Only one combination possible. So combining A with B would give you AB, which would be useful in one puzzle. But if you had combined the same object A with object C, you would have got AC, which would have been helpful in a different puzzle or part of the game! Once you combined A with B, you couldn't use it again to combine it with C and vice versa. This is another thing that would greatly help the replayability of the games. Of couse, this is only an example. Many variations could be made here. All you need is imagination.
8. Interconnectivity between episodes - think how nice it would be if you could play episode 1 of a game, then get episode 2 and load your save game from episode 1! You would still have some of your inventory objects, which would grant you access to special areas in the level, areas that you couldn't normally access if you had only the second episode (because you wouldn't have the savegame with the items from the first episode). This is a little tricky, because obviously you don't want the players to get stuck in an episode just because they didn't buy the previous one, but it could be made so that the players could access bonus areas or earn some bonus items like trophies, posters, different decorations or clothes that would be visible from then on in the "main hub" or starting area or the game. Like a bowling trophy that you could unlock if you had an item from the previous episode. And then, the trophy would appear on your table for the rest of the season, with some new items added each episode. Such a thing would seriously give a feeling of progression and character evolvment. This would also be a huge bonus for loyal players and an even stronger reason to buy the full seasons. And then, for the really loyal players, who have gathered all of the special objects, an ultimate achievement should be unlocked for completing the season 100%, along with some bonus easter eggs or such.