Nice read, there are some very good suggestions here are but some work better depending on what type of turn-based system you have. I'm not sure if the author had a specific system or game/series in mind but I would guess it was written from a point of view of a character can only attack once per turn and not based on action points. Here are a few examples of what I mean by different types of turn-based systems:

- No movement, just use weapons, magic, items (eg. Final Fantasy)
- Characters move in order of some initiative/speed characteristic, then they can move a certain distance and then can attack/use magic/use item, etc. (eg. Shining Force)
- One side moves, then the other, the player can chose the order of which characters to use when it's his turn (eg. Fire Emblem)
- One side vs the other where each character has a set number of action points, can use an action point, then do an action with another character and then come back to previous (eg. X-Com)

There are obviously other ones and other of these combinations, as far as I can see Original Sin is a combination of these. I'm not exactly sure how it works though. Am I right in thinking it goes in order of initiative and each character uses all their action points and then it goes to the next character? Or can you attack with one character, then use the other, and then finish using your action points with the first?

I have not played X-Com for years but I don't remember a grid overlay and I thought that was fine.
I also don't think at least 6 characters is always suitable. Space Hulk also had an action point based system and I generally rathered missions with 5 Marines best. Will less characters you feel more for each one. I think for Original Sin, if your character can attack a few times depending on their action points this really changes the optimal number. I think 2 or 3 should be fine and 6 (i.e. two henchmen and two summons) could be a bit too many but we'll see.