Hmmm... Drawing arbitrary lines on the map is what got the likes of Syria, and all that kind of jazz, in the first place.
Then draw non-arbitary ones. Look at what people live where and draw accordingly.
Right, I'd want to draw actual lines that make sense. There's simply no rational argument that Syria is a unitary territory.
The point, from my perspective, which goes against both your perspectives (agreeing and disagreeing) is that what is arbitrary can also be considered arbitrary. "Our people have always[1] lived here, you must formalise this as part of our territory." "Our people always[1]
did live here, you should not now formalise it as part of their territory, just because they got away with an expulsion, last year/ten years ago/a generation ago/following WW2(/WW1/Napoleon/Alexander The Great/The rise of the Chin Dynasty)."
Populations have long been ragged round the edges and blended (prior to actual fences/walls/deathstrips applied to more or less arbitrary territorial limits). That stretch of fertile land might definitely be Nation A's because Nation A has (on balance) more historic and contemporary claim to it, but then the river changed its course/relative distribution of flow slightly and Nation B has good standing of their own to insist upon the modern body sticking to the thalweg (never mind the logistical issues where it now needs a new river crossing just to let Farmer A reach his historic family orchards that are (by choosing river-boundary definitions, but adding on such documented ownership rights as they are) exclaves that now would seem to have better access from Farmer B's homestead in the neighbouring territory.
Actual mountainous watersheds are probably fairly firm (give or take a tussle about where upon the flattened saddle of a pass to set down the palings, concrete posts and/or barbed wire), if each side of a different people. Ignoring those times when (by marriage or conflict) posession was unified and basic trade led to social union prior to a later split (by uprising or divided-inheritance at the ruling class level) which kept the waters just a bit too much muddy to satisfy everybody.
Though it doesn't primarily redefine Syria itself (above prior and more recent territorial redrawings), one can imagine that Sykes-Picot could be either utterly dismissed, in favour of the concerns that Sykes and Picot were effectively disenfranchising with their scribbled lines, else that SP should be taken as a more legitimate delineation than various subsequent changes (is at least a fall-back option) or yet otherwise go with later changes (Mandate/post-Mandate revisions of claims) or even practical considerations from now (sprawling multigenerational refugee camps must now officially belong to whatever group(s) find themselves born and living within them, in a partially similar and partially flipled copy of the Gazan internally-displaced communities).
All decisions, and even the intent to
make such decisions, are going to be contentious. I've not been giving much attention to the shifting tides in Syria, but the typical pattern of 'government' control, official opposition/revolutionary control, splinter/cross-border factions, whatever IS claims to control/etc makes pretty much any partitioning exercise fairly problematic. Traditional cultural, geographic and de-facto (i.e. consistent military/force-of-arms) control boundary lines don't easily overlay each other, and even the best compromise will create a gerrymandered/Escheresque tiling of territory that would be (proportional to population) far more disruptive than India's partioning, harder to police than Cyprus's or Korea's dissections, certainly more than comparable to Sudan's experience and possibly even put the non-Syrian territorial changes around Israel in an almost trivial light. There'll always be people left in the 'wrong' places, or perhaps convinced (beyond reason) that they are in what should be the right place.
It would almost be easier to just extract everybody, draw some actually arbitrary lines and then reintroduce everybody into areas chosen so that they are
equally misplaced (all who previously claimed the land that anyone now is upon are on are now busy setting themselves up on land that only totally different people previously had a regional interest in; fresh start for all, in what's basically one mass and universal house-swap).
[1] FCVO "always", both provable and unprovable.