The trolls have been largely united by the events of the Battle for Azeroth -- the Zandalari has joined the Horde, and this gives them tremendous lasting influence, political, cultural and religious over the other troll tribes elsewhere. Now that the Darkspear and Zandalari are allies - perhaps they could together convince the Amani trolls of Lordaeron and Stranglethorn to become members of the Horde, ending their rivalry with the blood elves once and for all? There was this Horde quest during the Battle for Stromgarde:
The bloodthirsty Witherbark tribe has always refused to join the Horde... and now that the Horde holds Arathi, we will succeed where the Alliance has failed. Drive the forest trolls from the Highlands.
If the faction war is to continue - and I understand most people are tired, but again there are still massive tensions and unresolved political and territorial issues - the Horde might attempt to gather the remaining troll tribes and enclaves -- uniting them under one massive troll empire, which would surpass even the Zandalari Empire in the glorious days of old, possibly even the Kaldorei Empire.
The trolls were never truly united in the past, not even in the ancient times - but they could be united. It would literally reshape the course of history. And a united troll empire would be strong enough to stand against any outside threat or enemy.
Of course, for faction balance...the Seven Kingdoms, I suppose, could unite. Four of the seven kingdoms have been successfully restored - Stormwind, Stromgarde and Kul Tiras, and Gilneas is firmly Alliance-aligned, but while its territorial status remains very vague and unclear, its immediate neighbors (Tol Barad, Southshore, Fenris Keep and Shadowfang Keep) have apparently all been retaken by the Alliance. Perhaps Dalaran could rejoin the Alliance; the blood elves and nightborne could remake Suramar as their own flying city over the Broken Isles, complete with telemancy networks to the other continents. Alterac's remnants could try to rejoin the Alliance in wake of an reunited troll empire as their neighbor. Lordaeron, I'm not sure, the Keep is still blighted - perhaps there could be a split, an internal civil conflict with the remaining territories divided between them, with the living of Lordaeron fighting for Turalyon, and the deceased fighting for Queen Calia Menethil and her troll allies?
P.S. The night elves could also possibly attempt to recreate the Kaldorei Empire. Perhaps they could reunite with the Broken Isles night elves, and other neutral settlements. But as Tyrande is very unlikely to retain her powers after this expansion if she even survives, and their population is implied to be so low, that even all their people working together could not match the Horde-aligned trolls in terms of raw power, let alone a united troll empire standing together.