.

Seconded. This is crucial for it being a good game, and should be one of the first things done once the interface is in a good place and the rules are sorted out.

The investors can fundamentally alter some of the game elements/add special exceptions/override base game rules -- this could result in big changes to the underlying code driving this game.

I would very much suggest (without knowing what the code looks like) that the less impactful time to do this is while in alpha instead, where potentially breaking the game is waaaay less concerning.

I think I agree with stefano that this can wait until Beta, given that stefano has already said he's not going to add any expansions in. Some of the expansion player characters are more complicated. I'm just going to go over the base game investors:

Bill - No special powers, just a lot of money
Bernie - Reads trading fees as positive instead of negative. Easy to code, I think.
Cramer - Gets a free action every turn of move a stock up or down 1, easy to code, I think.
Donald - When buying a pile, pay the value one space behind where the meeple is. So, just have the game read his bids as values one step lower. Pretty easy, I think
Deborah - gets to declare dividends for 1 company per turn. A little weirder, but shouldn't cause problems.
Mark - Move any one card to a different pile before bidding starts. I think that should be pretty easy?
Graham - Get +$1k for every stock sale. Very easy.
Martha - Can look at any face-down forecast card during bidding. This is maybe the most complicated one, because there's currently no mechanism for it that you can just move to this player. But even so, I think it'll be fine?
Stuart - Place both cards face down during supply phase. Easy enough.
Warren - can look at all the face down cards in one pile before bidding. Ease of this one is dependent on whether you can currently look at the face down card you put yourself. If yes, then should be easy. If no, then may be harder, because you have to write that feature from scratch. Also, limiting it to one row.

Anyway, I think it's 8 out of 10 that are easy enough, just giving a specific player an ability that already exists elsewhere in the game. Martha and Warren may be more difficult, but not overly so. So I think it's fine to wait for Beta, though of course I'd love to play with them as soon as possible.