Next step, I'll tackle 4 more (and again) for myself:
Germany: @Grimlin, Willkommen, du hast gute Ideen, aber falsch begonnen (Good Ideas, but you started the wrong way). Rather than proposing single UU's, first set a theme for the civ. For the Germany of the OP this is warmongerer from the start who can "ignore" the World Congress and has a late boost. Conquest often means not having many CS friends which equals few votes in the Congress which means Resolutions contrary to your interests get passed. Germany should turn the tide here. (And @Ahriman, the Diplomatic Genius of the Germans does refer to the unification through Prussia which required lots of it, even though it did bring Prussia to the brink of defeat in the 18th century (read up on Old Fritz f.e.). But it can also refer to the current situation where the Germans became the "Kings of Europe" even though they didn't really want it (this time

). And then there's the Berlin Conference of 188something on Africa. Metternich was Austrian, but we know that distinction is a bit fuzzy as well

) Shall I go on?
I like that theme and I don't want to go back to a Engineering theme. That one can become quite bland (I haven't played Babylon once because of that). I could see the "Elite Unit"-UA get passed on to Sweden (I don't like the farm UA), but it does fit Germany. I feel we just need to find a good way to translate that "can ignore the World Congress" to a Unique (Building)
Arabia: See, we forgot the theme in the discussion above. I agree that a Tourism bonus isn't the best, why don't we have a religion+money civ after all? The Desert bonus may be situational, but it can be quite strong. I don't think focusing it on Land Trade is a good idea (a) it's not historical, b) it's more flexible the other way and c) it isn't a particular enticing way of playing the game, such as Polynesian Exploration is...
UA: Trade Routes (only them) treat desert as roads (= increased range through desert) + Desert Movement for Units (to protect them in the desert) + religion spreads through all trade routes
UB: Madrassa or Bazaar: Faith (or Science for the Madrasa?) per trade route to this city
UU: Camel Archer stays
Fixed the "no religion bonus", but didn't make it particularly about gold
One problem though, we changed Arabia from a wide civ in GEM to a Tall one (contained to the deserts?). Or is that a problem?
The thing with the "desert acts as roads for trade routes" is that is that it increases the range, but not in a radius, but selectively... I think the extra range primarily gives more cities = better choice for a good route. In the early game, that's mostly too unsafe though. The extra gold should be minuscule for Arabia here though, no?
America: What people seem to miss with the Pioneer is that it is needed with the UA. If your settler moves faster than the warrior or archer accompanying it, the extra speed is for naught. But then again, I do not protect them directly as much as in civ4 since add sight and 1 Upt often means they're safe already.
On the other hand, does America really need another boost. Fast Workers does mean they can move onto rough terrain and start working right now, right? So that's good, extra sight is undervalued, we already know that. 1 happiness per city is strong as well. And the Space Factory can be a real killer, we know for experience. I'm okay if America gets a "dud" UU...
The civ seems solidly designed to me, even though the extra worker working speed proposed above might be a good replacement for the passive happiness (=faster linked up luxuries). But it isn't as flexible as the happiness (settle where YOU want).
Celts: See the Opening Post. Thal asks us for ideas here especially!
I felt the forest-faith bonus was a bit forced from the start here + the Double Yields from NW fits more with a widely early expanding civ. The Celts should do warmongering with the Pict Warrior... I could see the double Natural Wonder being moved to the Shoshone (and the free tiles toned down there...). But we do need a theme.
Religion is fine, but it is lacking a bit on its own. The Celts should definitely be wide (no really big cities). Looking over the list, we could use a Religion + City States civ, can't we? That's not necessarily wide though. I'd rather see them as a bit of religious warmongerer: My try:
UA: Druidic Lore (Faith from Vegetation, Forests give a healing bonus to all Celtic Units)
UU: Pictish Warrior (Spear with faith-per-kill and foreign land bonus = rush)
UB: Abbey (1. Buyable with Faith - from the start -, adds 1 of every yield (except tourism) = something to burn through all that faith, the bonus can be adapted of coures), or else: Earlier Opera House? UI buildable next to bonus ressources adding 1 food, 1 production and 2 gold?)
@Thal Can you add to the Opening Post the start biases and the avoids as well, might be worth for the discussion of the civs. I have seen it argued for example that a Tundra start is a bonus (no civs to the North/South) as well, but that might not necessarily be true for a warmongering Sweden (=less targets...

)