To start, here's two ideas I've been mulling over. In light of my love of
modularity, what about
Modular House
Now, I know what you're going to say - "but they have modular buildings already - isn't this just a ripoff of that?" To whit I reply "I don't really think the modular buildings are very modular." Sure, you can sort of re-order the floors, and you can connect multiple buildings together, but that's not modular enough for me! What if they were comprised of multiple smaller 'room' modules that were sold individually, or as combo packs?
Think in terms of
3315 Olivia's House (an awesome set, btw, which was partly the inspiration for this concept), where you can rearrange the individual rooms. I would LOVE the ability to add more rooms to this set so my daughter can have a den, a dining room, a half-bath, etc. Sure, I could buy another copy of the set, but I don't need
all the same pieces, just
SOME of the same pieces.
So, you could have a series of sets that belong to a greater collection like "Living Room", "Kitchen", "Front Door/Hallway" or you could have a "First Floor" collection that includes all three. The design would be such that you could build them each and arrange them in any order you wanted. Alt builds allow you to make them a corner of the house, or interior-only. Buy another "Front Door/Hallway" to use as a side door (maybe there's an alt build included as a Creator-type 3-in-1 for a side door).
Creator Scenery/Accessory Packs
In light of the above idea, you could take the ideals of "Creator" and "Bricks and More" and put them together. Image a set with green baseplates, various brown and green slopes, and other accents (apples, flowers. Birds? Squirrels? Maybe a minifigure or two...) with a series of alternate instructions on building different types of trees - a "Tree Creator Pack" that you could use to augment your existing sets. Maybe instructions for a few combo models, like "buy two and build this oak tree", "buy five and build this California Redwood"...
You could do the same thing with other scenery type things, like lampposts, fences, etc.
Additionally, Creator Furniture packs to populate the modular house with different stuff. Alt builds for each one - "build this office desk, a set of bunk-beds, or this dining room table". Combo models here, too, maybe.
or 
smilies. Add some constructive criticism either way, if you have some. Like this:
