) Would adding a seventh ability add a lot of complexity to that part of character creation? Is it already a bit fiddly to take a point total and split it among six things?
No, but it should be something neither mental nor physical-- like a magic, essence, or similar ability.
Which brings me to a different point: they shouldn't be call
abilities, they should be
attributes.
2) Is there something magic about six for the D&D feel to you? (I mean they tried adding comeliness back in 1e). Or does the number of them not matter?
Nothing "magical" really except they stem from a fairly logical break up of the attributes people have: some are strong, some dexterous, some intelligent, some charismatic, etc. IMO they have enough distinction to set each apart, as where if you tried break them down further it might be too granular, if you group them together (like combining STR and CON) they might be too broad.
There are systems out there with 2 or even 3 scores, others with a dozen or more, any number can work IMO as long as each score is required and serves a purpose--hopefully for each PC regardless of their focus or role in the game.
3) Would it be bad or good if each class really had two stats that were particularly helpful instead of just having one that they really lean on?
IMO in 5E this is already the case if you look at the game as it is now.
IME this is the typical ranking of abilities
Barbarian: STR/CON/DEX
Bard: DEX/CHA/WIS (or INT)
Cleric: WIS/CON/STR (or DEX)
Druid: WIS/DEX/CON
Fighter: STR or DEX/ CON/ DEX or STR
Monk: DEX/WIS/CON
Paladin: CHA/STR (or DEX)/CON
Ranger: DEX/WIS/CON
Sorcerer: CHA/CON/DEX
Warlock: CHA/DEX/CON
Wizard: INT/CON/DEX
Unless you have a combat-lite style of play, DEX and CON are already usually the #2 and #3 scores, helpful regardless of class in significant ways. Most classes can have a score that is primary, allowing them to focus on these as secondary and tertiary, but a some classes (e.g. Paladin) also needs to have other abilities to focus on. This is why players sometime are disgruntled about such classes, because they feel their DEX and/or CON must suffer to be good at what they need to be good at.
Now, by retooling DEX and CON so they are "helpful" but don't feel nearly as essential, it helps mitigate this.
And then two more going for if they would break the D&D feel for you, and not necessarily if they would work in terms of mechanics.
4) What is your gut reaction to splitting Dexterity into something like Dexterity (Coordination/Aim) and Agility (Reaction/Acrobatics) ?
IMO all you would be doing is taking a score which is already good, and making it two scores that would both be highly desired. This falls into the "too focused" concept for the separate scores.
5) What is your gut reaction to merging Strength and Constitution so that handled both weight training and cardio type fitness?
And this would fall into the "too broad" category IMO.
Finally, I would get rid of ability scores in the current sense altogether. One thing I've said before related to skills in 5E is this:
Being strong doesn't make you good at athletics, practicing athletics is what makes you strong.
Being intelligent doesn't make you know more, knowing more makes you intelligent.
And so on.
It wouldn't be "D&D" really at that point, but something like having proficiency in three Strength-based skills would give you a +3 STR mod, having two Charisma-based skills would give you +2 CHA mod, etc.
I've always liked the idea, but never taken the time to try to flesh any of it out.