My personal stance is that halflings and gnome actually don't share that much other than being.My position has been to merge the two groups that are very similar and both partially lacking in long-term durable appeal--halfling and gnome--to create a single group with more options and stronger inherent flavor than either option alone. It's a rare example of me favoring an ever-so-slightly more minimalist position, but since almost nothing is actually lost (ghostwise halflings and svirfneblin aren't exactly dramatically different, and that's really the main "loss" here) I find it not much of a minimalism approach.
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Other than that, every species/race/whatever had four flavors for overall consistency. Hinnfolk (halflings + gnomes) also had four: lightfoot, stoutheart, cragstep, ghostwise. They're creatures of the land, tending it and listening to it--but those who come from deeper in the earth are weirder than those that come from its surface. Cragstep aka "rock gnomes" are eccentric inventors and tinkerers, but usually not dangerous. Ghostwise are sharper, wiser, but also...more than a little too close to the weirdness of the Underdark.
Halflings literally are small humans and that's the problem.
Homes are there on thing.
So in order to really together halfling and gnomes you would have to really lose a lot from.
That's why to me personally would be to make halflings more separated from the talking idea of hobbits them being pretty much small humans that are slightly more lucky into small humanoids that have their entire existence based around luck.
Halflings would have a access to the luck mechanic that the base game uses much has how elves naturally have spells.
So to me halflings would have a natural luck point and a sub species would have individual subspecies uses for these luck points. Light food would be able to gain a luck point when they roll a one and does ignore the one results. Where is a stout halfling would have the ability to spend a luck point in order to reroll any status effect affecting them .
Gnome can then stay as masters of craft either by tinkering or illusion as Rock gnomes and Forest gnomes do respectively. That's why I kind of gutted any other kind of gnomes other than deep gnomes who would have a dueling and mining focus on Craft.