There are so many themes they could introduce into TS4 through SPs, that with all the themes they could build onto TS4 at least 3 to 4 more years with just SPs and GPs and I would be a happy customer.
@SimsILikeSims... I like a lot of your ideas you mentioned, which a lot you mentioned they could combine in packs. Like your idea for laundry, household chores, and household clutter. A pack with a laundromat venue, a cleaning work station, a new skill- Cleanliness, and a new active career patched in before the pack was released- Maid.
As for packs like an Accessories pack... a Stylist pack where our sims could be a fashion stylist and/or makeover stylist would work really great for more accessories. I also think it would be awesome if they added the ability to craft and design our own custom earrings, necklaces, rings and bracelets by using gems and crystals our sims have harvested.
I do hope we get another Perfect Patio/Backyard themed pack this year but it being directed to more of a Pool Party setting with more swimsuits, new wet-hair texture hairs, standard diving board, pool slide, above-ground pool option, pool toys, floaties, loungers, etc..
With the Holiday SP I would just keep the free content updated from the pack we already have. Where we just received toddlers it would be great to have a Santa Suite and Mrs Clause Suite. A "Sit-On-Santa Lap" option for toddler and child sims to have Santa grant their "wishes". A new elf outfit for toddler and child sims to wear would be adorable. Maybe even add "caroling" for sim households to partake in going door-to-door in the neighborhood.
And your idea about the abandoned house/spooky add-on. I think that would work better if Ghosts got their own game pack with new derelict looking build mode items like boarded up window, broken glass windows, cloth-draped furniture, haunted portraits, etc. A new active career for normal sims like Ghost Hunting. And ghosts got an upgrade with their own perk system with their new skill being "haunting". Make ghosts more paranormal with the ability to scare sims to death. A new reward trait from Aspirations could include "Third-Eye" where normal sims who have the third-eye could befriend ghosts and be immune to their scare tactics.