If they did a Get together refresh what I would most like to see would relate the club feature:
- Ability to manage and edit all clubs from for instance the manage world view
- More clubs per sim
- More club members per group
- Make sure all activities from various packs are included in the things sims can do (and more activities in general)
- Allow clothing in a group to be set to formal, everyday, sports... not only the categories that are now
- Allow toddlers to be a part of clubs
- Allow more ways to filter memberships (unemployed, student at university, student at a certain university, a member of a certain family (related to a particular sim or sims), only sims who live in a certain world...)
- Allow for easy invitations to events by not just being able to filter by clubmembers in general but a specific club (invite all McWilliams family)
- Allow for automatic filling of clubs when sims meet the requirement (on club basis so it can be turned on and off for each individual club - as there might be clubs you want to fill and those who you want to have more control over).
In addition to this, I think more instruments and using it as a sort of "band pack" as @Mak27 suggests would work well with the intention of the pack, but then we'd need more performance actions, and more items for performance (stage, speakers, smoke machines). Not that I'd mind that at all.
More pool objects would be welcome in general. Slides, jumping board (in addition to the tower). Pool party event would be nice, though that might be more seasons. Talking and drinking while relaxing by the pool. Pool bar would be fun.
More games that are played together in general - though I would love those in base game as well.
More games for kids that does not include toys!! (though again, put at least some of those in base game).
New entertainment careers would probably more likely go in a Get Famous refresh. Also not sure if roof gutters has any connection to Get Together. Can't see why that wouldn't just be a build kit or in base game.
I would also love to see a pool table like @Sallycutecat suggested, and various form of dancing together like @LeGardePourpre suggested (though slow dancing should be a base game update)