and the rebellious foederati settled in Histria, Flaminia and Liguria mainly.
I'm not too sure about this : mutinees asked for a very large part of Italian fiscus, and were dissatisfied with their settlement in Liguria.
Now, settling them in eastern Italy makes a lot of objective sense on this regard, strategically at least, but it wouldn't met their demands. If they are to be at least negotiated, I'd rather thee the hospitas and fiscus usufruct being localised in southern Italy, maybe Adriatic italy up to Ravenna.
Now I do think these demands (a third of Italy's fiscus) were unreasonable to begin with, but Orestus and his blank check are to blame there : maybe a good PoD would be him not being this formally generous thinking mercenaries and federates would "get the joke".
This Roman guy (Ecdicius Avitus buttlerfying death)
This is really unlikely : his father's claims were never supported by Italian senatorial elites, which provided the basic grounds for Gallo-Roman being at odds with whoever ruled Italy at this point, and if his choice as commander by Nepos is understable giving that he had to fight in Gaul, he wouldn't have any real support in Italy at this point. If he survives, I'd rather seeing him playing a role in Gaul under Barbarians than in Rome.
Anyone desesperate enough to propel him as emperor would be quickly disappointed when it comes to political results.
I'd rather see whoever rules in Dalmatia at odds with Constantinople being chosen instead, or maybe Nepos returning in Italy with the support of, say, Odoacer and then being replaced by a rando.
From 74 to 91 ITTL Theodoric gets a very different approach of the diplomatic game and build no strong ties into every arian kindom around
Theodoric's diplomatic approach is essentially tied to his role as effective imperial lieutnant in Italy :
how he recieved imperial regalia IOTL stresses this. One couldn't rule Italy and not claim some sort of dominance in western Romania as first among equals.
Butterflying Theodoric's kingship in Italy is an interesting change, but whoever would get it would at least try to act the same.
Before he can launchs the campaign in name of Zeno, he strucks dead and the ostrogoths lays outside Italy until 502 (and a very annoying sucession struggle)
At this point, I'd expect Constantinople to change their stand : they wanted to curb Odoacer's ambitions and leaving Dalmatia under his dominance was a big no-no IOTL. Even with the changes already happening ITTL, I don't expect ERE to accept that an usurper (would it be styling himself as emperor) would keep the region. If the sons are too young, that's too bad for them, but a king would be supported to do the job : either another Amal, either a non-Amal, but someone.
I'm wondering how the gothic society will emerge without the "great man" work of Theoderic and with a delayed and more practical clash of costumes and laws
Honestly? Not too dissimilar than how it happened IOTL, probably, at least for what matters Gothic Italy : you'd have a pretty much romanized already elite being integrated within the frames of post-imperial Italy, and acting as primates of western Romania up to a point.
This being said, the delayed conquest would mean that Franks would stregnthen their dominance in the region earlier and possibly deeper than IOTL. Without Goths to act as a counter-power in Central Europe thanks to the aformentioned primacy and power in Italy, or to play as arbitles against Clovis' conquest...Well, there's a fair possibility Gaul would be unified earlier than IOTL, including Septimania and Provence, maybe Burgundy quicker than IOTL, while dominating kingdoms as Thuringians or proto-Bavarians earlier than IOTL (historically, the collapse of Gothic Italy allowed Franks to replace Gothic influence there).
In fact, Constantinople could find convenient enough to search the Frankish rear alliance as IOTL, but not as much against Goths than whoever rules Italy.
While your ideas and proposed changes are really quite plausible or workable, IMO if they have to lead for a dynastical wank up to imperial establishment (which would be relatively problematic, for various reasons), it would really be more so for Merovingians than Amali.
(as visigoths in hispania? Idk)
Most of the clashes came from a pretty much aggressive expension in the late Vth century : Alaric II attempted to draw significant political and religious compromise policies, but defeat face to Franks and the disappearance of the Amali dynasty in Spain provoked the appearance of an anti-dynastic and potentes-heavy Gothic Spain. The problem was much less laws (Barbarian Laws are essentially Roman laws on Barbarians) or culture (materially undistinguishable) than political in the broader sense.
EDIT : By the way, welcome on the board!
EDIT : If you want a significant enough departure from what existed IOTL in Late Ancient Italy, you might want to get inspired by Genseric's exemple : while most of Barbarians large settlement in the Vth century didn't involved requisitions or expropriations, Vandals confiscated a lot of lands in the "sors Wandalorum" in Africa Proconsularis, making a quick and radical change there.
Now, Vandal Africa wasn't cut from the general post-imperial ensemble, but Genseric's political stance on this and on religious grounds (which were, while not fanatical, definitely exceptional among Barbarian kingdoms) could be an inspiration for an alternate Barbarian state in Italy.
Let's imagine that Goths (or whoever takes the lead in Italy) are led by someone either inspired by this exemple, or inspired by a figure like Severinus mixing political role and religious prestige, and doing this and it might be a good step for a different regional set of specificities if not a whole departure from what existed IOTL.
It's not the most plausible setting, I agree, but as a tought experiment and a prospective TL, it could be interesting : such stuff would probably antagonize both Barbarians and part of Romans (mostly expropriated ones and Constantinople) as it happened with Vandals, which is something that you searched (even if it wouldn't make social evolution radically different than in Barbarian kingdoms ITTL or IOTL).