1) Right to Rule doesn't add any new wonders, so here go civs which already have assigned wonders. All of those four civs also fit the theme of "right to rule" - imperial power and conquest. Assyria and Goths are in, because there are confirmed wonders in the game which are frankly not too impressive
choices typical when civ7 is desperate to figure out any wonder connected with a civ. No way we would have got "Tomb of Theoderic" as a wonder if not for this reason, and "Dur Sharrukin" is simply one of Assyrian capital cities, so again, somewhat desperate
Additionally, Germanic civs being formed out of Rome and lack of Mesopotamia in the game would feel incredibly lame, so Goths and Assyria solve those lame issues in favour of totally badass solutions.
2) Also, Germany and Russia are going to get there because a) Thematic reasons b) Frederick and Catherine in one DLC are super badass marketable combo c) One cannot have civ game without those two civs.
3) Goths go through Normans to Germany, and Russia is derived from Mongols, unless you alse get second DLC, where they get Goths->HRE-Germany and Greece-Byzantium-Russia as more sensible paths. Awkward, I know, and may cause some anger as seems blatantly engineered to convince people to buy both DLCs at once
but you gotta admit it's a smart economic move, just like staple Aztecs and Babylon being day 1 DLCs in the previous games
4) Also yes, Russia goes from Greece and Byzantium, as it is the laziest simplest path possible with one less slot required than the political minefield of say "Slavs -> Kievan Rus -> Russia"
5) Crossroads of the World is the one DLC from those two which actually adds new wonders, so previously wonderless civs go there.
6) Ottomans go there for Middle East region to get modern civ just like all other regions get one.
7) Byzantium goes there to provide a) The only sensible path for ancient Greece b) Second and better path for Russia c) Alternate path towards Ottomans, d) Being incredibly convenient civ for providing decent links for a **** ton of civs, while also being immersively ruled by a lot of outside rulers without need to get its own emperor (Augustus, ancient Greek ruler, Catherine etc)
8) HRE goes there for Germany and Germanic civs in general to get full evolutionary line akin to Normans leading to France and Britain, also it is worth noting that we have Charlemagne confirmed as a leader
and it is again incredibly conventient civ for future linkages
9) All those three civs as well as Polynesia or Silla perfectly fit its "Crossroads of the World" theme.
10) Silla is confirmed because, let me repeat my favourite brutally honest argument, Seondeok's Bell is too lame for a wonder to not serve the singular purpose of filling the required void.
11) Silla gets to the game to provide full set of six East Asian civs, so two players can play East Asia simultaneously.
12) Tonga gets to the game to provide progenitor for second era Hawaii, which then historically turn into USA (or Maori in the future).
13) Britain gets to the base game because we can't have base civ without it
14) Britain also gets to the base game because it would make no sense to make the whole Norman business without trying to square one civ slot serving as a progenitor to both it and France at the same time.
15) The reason why I expect Polynesia or Hawaii and Silla in particular to get new leaders is because those are quite distinctive cultures while also offering some great options. Meanwhile HRE already has Charlemagne, Byzantium can have Augustus or Alexander the Great as a leader, and Assyria, well I would love for it to get a leader but it can live with say Xerxes.