There are lots of variables & outcomes
Best thing to happen right now would be a total ceasefire & a UN enforced no-fly zone, leading to full de-escalation on all sides
but in order for that to happen Assad would need to be given some kind of deal to go into exile or else be tried at the Hague.
He's already refused at least 1 or 2 exile offers I think & to get him to the Hague ...well
Russia & Assad are allies. If Trump is serious about 'regime change' then he isn't going to get past a Russian (or maybe even Chinese) veto against joint UN action in Syria so what's he gonna do?
I think this missile strike is largely symbolic, designed to first of all show that America cares/is responding & possibly with the added hope that Assad will feel threatened enough to accept some kind of exile deal, probably in Russia.
There's a few problems here though - even leaving aside the whole enormous question of what fills the power vacuum & the complicated situation of ISIS, overlapping rebel groups, Iranian influence, Kurdish autonomy, Turkish military, Russian involvement & Israeli anxiety (!)
... Assad has already shown that he gives no fucks & losing an airfield is no biggy. Trump's missile strike will have done jack shit to scare him
However, what it has done is land a domestic spike of approval for Trump ...and as we all know, if there's one thing Trump loves & craves it's approval
So right now the worst possible thing that could happen for the people of Syria ie the escalation of conflict is what looks most likely. If Trump commits fully to regime change in Syria then you can expect the UK, France & Israel to be coalition partners. Turkey will be in the fight too but with conflicting interests, especially if the Kurdish forces are the defacto coalition troops on the ground.
Even the most optimistic version of this scenario would be carnage for Syria - especially if (like in Iraq) there is no clear post-conflict plan for peace, conciliation, & rebuilding.
Also, depending on what positions Russia, Iran & China take this could go very bad indeed. WWIII bad
Perhaps ironically, the biggest hope lies in Trump & Russia actually having a good relationship and being able to get around the table and do deals - for Assad's exile while protecting Russian interests, for co-ordinated effort on ISIS, for transition to a stable Syrian political structure & investment in infrastructure rebuilding (which is an area the Chinese would want to be unvolved in presumably)
Now, depending on your point of view, this kind of multi-lateral negotiation of international parties is either the recipe for stability & development in the region or the same old imperial/superpower vultures carving up the carcass that led to this situation in the first place and guarantees it's perpetuation
Short versio:. Who do you trust ? Trump? Putin? Assad? Erdogan? Netanyahu? Iran? ISIS?