In my view the most fun way to play is as a horde. You can re-horde in midgame and be stronger than ever before! Horde size depends mostly on the population of the city you horde from. So settling your first horde, then raising a second one in Rome really rocks!

It seems enemy hordes do not usually attack each other unless one of them blocks the path of the other. I have recrossed the map multiple times, and come close to the Vandals and Huns many times without a fight. It helps to have a diplomat nearby to ask for a ceasefire, but this is not always necessary. It also helps to save often because sometimes you may block them and provoke a fight, but if you stop one space to the left they leave you alone. Also I never maintain mercenaries with my horde. With zero mercs there is zero expense, and all the time in the world to maneuver.

If you kick the nests of the lombards, burgundians, and franks you can force them to become hordes. They always seem to accept a ceasefire the first turn they become a horde. There is little danger of squandering your horde by attacking them. Once loose, they will turn WRE into anarchy.

As the Goths I find it best to spend about 9 turns milking the homeland, then horde. Like many others i take tribus lazygens and build some forts. I recommend enslaving this population because a bigger pop in dacia will give you a bigger horde.

I build blacksmith 1 and 2 in dacia, and recruit 9 raiders there. In tribus lazygens i build a stable and 5 lancers. when the lvl2 blacksmith is finished i retrain all my forces including family members. At this point I'm bankrupt and it's time to horde.

This early in the game Greece is poorly defended, I sack it forthwith, but do not settle down. Then I move west sacking along the way. I think I hit the lombards on the way as well. I settle in Rome and two other Italian cities. I spend all my cash on military units. Let the other two italian cities revolt, then horde up from Rome. At this point I have 9 full stacks. 7 stacks are horde troops, 2 stacks worth are tricked out hard core regulars.

I've tried settling one city, disbanding just 1/3 of my horde then hording back up. This seems to always result in a slightly smaller horde. There can be an advantage in doing this. First you can give the other 2/3 blacksmith upgrades. Second, you can convert slave spearmen into more useful troops. Have the army that takes the city be all slave spearmen, and place the rest of your slave spearmen close by on the map. Have valuable horde troops be far away. The 1/3 that disbands will be almost all slaves. Keep the city at least one turn, then horde up again. I found that disbanding about 16 units, I got back about 10.

At midgame Europe is complete anarchy. It will take a long time to see who prevails. The Saxons and the Franks are early favorites to form the first big empires. So I go sack Denmark and stir up the Burgundians. If the Allemani are still alive I let them alone as well. Chaos is served by leaving as many competitors as possible. I really can't decide if it is worthwhile to attack the Franks or not. Either way they are liable to form a major empire in Gaul.

With Greece wounded early, the ERE will inevitably succumb to the Sassanid empire. There is only one faction in BI that is predictable, the Sassanids. Therefore it is vital to nip their plans for world order in the bud. I hike to the far eastern extreme of the map, then south. This is a long hike but it takes little time, as there are few battles and turns are not complicated. The Roixolani may be trying to invade along the eastern shore of the black sea. I go around them. Sassanids main forces are concentrated on the ERE in the west. I sack all the poorly defended Sassanid cities I can find. Then finally find a homeland.

My choice of homeland is Antioch, Jerusalem, and Alexandria. These give great cash flow! And they are built higher on the tech tree than the Goths can ever match. If the ERE is still near Antioch, its better to find an alternative. Because it sucks to be between ERE and the Sassanids! The Sassanids and ERE will never stop fighting each other unless you get between them. Benefit from their mutual attrition.

I've used this strategy with several horde factions.
Goths have good anti-clibinari units compared to most factions. By the endgame several factions bit the dust, i know not how or where. Somebody in the west is ending the chaos and forming a large empire. I've often seen the saxons or the franks become powerful in the west. Any of the horde factions might have taken up residence in greece or italy, but they are menaced by all the remaining factions so not a great threat. I've seen the Vandals secure in Spain and expanding out. Once I've seen the Roixilani become masters of most of Germany and Russia. I've seen the lombards rule Italy (but not for long). I've seen the Slavs and the Ostrogoths emerge as major empires, or sometimes they die with a whimper.

Playing a globe trotting horde is a great way to play! Every game is different. The late game map bears no resemblance whatsoever to the starting map, and there is no telling who will be where and how strong.