Canon

Canon in Baldur's Gate universe usually revolves around the so-called canon party that travels with Gorion's Ward in-between the events of Baldur's Gate (game) (or Baldur's Gate: Siege of Dragonspear) and Baldur's Gate II: Shadows of Amn.

Canon party
The canon party for Baldur's Gate is obvious from the way Baldur's Gate II begins. Gorion's Ward was captured along with Imoen, Khalid, Jaheira, Minsc and Dynaheir, so this is the canon party for the first game in the series, and pretty much set in stone.

The canon party for Baldur's Gate II is a bit more complicated. Obviously Imoen/Yoshimo, Minsc and Jaheira are in, as you gather them all in the Irenicus' Dungeon. Yoshimo is an instant Thief replacement for Imoen and his "personal quest" requires him to follow the party to Spellhold, so that's the core canon party in.

A good case could be made for Aerie for several reasons. One, she can join the party in the Waukeen's Promenade, which is still technically before the game officially begins and after what will likely be the first major quest in the game for most players. Two, she is an arcane caster replacement for Imoen. Three, she has a good chemistry with Minsc and bad chemistry with Jaheira (bad banter is good too). Four, she is a romance option with possibility for Gorion's Ward to have a child that can actually be seen in-game.

One could argue that taking what amounts to a naive teen for a deadly mission to rescue your sister (whom she has no reason to care about) from a mysterious, powerful mage is silly, however – all the other NPCs have a reason to follow you: Yoshimo is forced, Jaheira and Minsc are old friends and lost loved ones to Irenicus, Imoen... is Imoen.

With Aerie, that still leaves one slot open and various theories were created as to who should be filling it. One opinion is that the sixth slot should remain open for two reasons: one, it will allow you to do every personal companion quest in Baldur's Gate II without changing the core party composition (i.e. take in Korgan; take in Keldorn, do his quest, kick him out; etc.) and the party have already have all bases covered class-wise. Second, that position would be filled by Sarevok in Throne of Bhaal. Sarevok can be considered in Throne of Bhaal canon-ish as well, as he is the only one with a personal companion quest – if it can even be called that – in that game.

So at the end of Throne of Bhaal, the "canon" party would be Gorion's Ward, Imoen, Jaheira, Minsc, (Aerie, optional) and Sarevok.

Canon Gorion's Ward
Now for the other variable: who is Gorion's Ward most likely to be?

Race would be limited to Human, Half-orc, Half-elf or Halfling. This is because the Baldur's Gate Prologue narration establishes Gorion's Ward at about 20 years old, and these are the only races which reach maturity in that amount of time (Dwarves and Gnomes: 40, Elves: 110). Note that there were no Half-orcs in original BG and their description says that they're tolerated in Amn. Baldur's Gate doesn't take place in Amn so picking a Half-orc might be a role-playing faux pas.

Alignment would be Good or at the very least Neutral. Obviously, the canon party in Baldur's Gate is Good, the projected "canon" party in Baldur's Gate II/Throne of Bhaal is also Good, and the games are heavily skewed towards being Good in general, with evil being almost an afterthought addition. Also Gorion's Ward was brainwashed by Gorion to be a hero ("raised on a thousand tales...")

Gender is open, though the "canon" BG2 party contains no romanceable males.

Class. Here's where it gets really interesting:
 * Sorcerer is probably top tier pick; the very description of the class mentions "powerful bloodlines" (hey, that's us), Candlekeep is a place full of scholars and your foster father himself is a sage.
 * Mage is another great pick, for the same reasons except bloodline.
 * Fighter is good because man, there sure is a lot of those plate mail wearing Watchers around.
 * Berserker is Fighter + Bhaal bloodline, so even better.
 * Paladin/Cavalier is Fighter + Gorion's brainwashing.
 * Thief? Well... Imoen is a Thief so at least it's possible, though she seems to slave away for innkeeper Winthrop, not learn under the sage Gorion.
 * Cleric is also not completely out of the question, though basically only because "Candlekeep is a scholary place". Before Enhanced Edition Gorion could cast Cure Light Wounds, so he could be considered a dual Cleric/Mage himself.
 * All the multiclass combinations of the above.
 * Dual classing to just about anything is legit, with extra points for a Cleric (learning to control your divine essence after you learn about your heritage or something).

Finally, we have the canon party class compositions to consider. Baldur's Gate's canon party has one front liner (Khalid), three second liners (Minsc, Jaheira, Imoen) and one third liner (Dynaheir). Baldur's Gate II's canon party has one front liner (Jaheira, promotion thanks to Iron Skin), two second liners (Minsc, Imoen/Yoshimo) and one optional third liner (Aerie). In Throne of Bhaal another second liner appears (Sarevok). In other words, the canon parties in general lack a solid "tank" which would point towards Gorion's Ward himself being a martial class.