Hi, everyone. I've done a bit of thinking about this one, and I really feel like 'advanced' classes can go one of two ways...
1) You reach a certain level (let's say, level 50 for simplicity) and are asked to choose a 'specialization'. From that point forward, your 'class' is split into two (or more!) and they level separately with different weapons and abilities.
Or...
2) You reach a certain level (assuming it's 50 again) and you undergo a 'difficult' quest to unlock other jobs... similar to the way you unlocked 'advanced' jobs in FFXI... except it's higher than before... or perhaps they'll add it at a lower level (when they DO add it).
Or, there's a third scenario... it could be a mixture of both!
As far as right now, I see CON+THM in a very clear way.
CON: Direct effects. Direct healing, direct nuking. (The two most obvious examples are WHM and BLM.)
THM: Indirect effects. Buffing, enfeebling. (The 'main' en-feebler would obviously be a RDM, but what about someone that specializes in buffing? Perhaps a BRD would be the best equivalent?)
A WHM specializes in healing, but could still do a little bit of damage... a BLM specializes in damage, but could still do a little bit of healing. Same with an en-feebler and buffer. A bard was obviously best at buffing, but they had a few important enfeebles too... a RDM was great at enfeebling, but had some irreplaceable buffs...)
The most important thing to note here is the fact that like with all things SE produces, these classes are heavily dependent on each other. Yeah, a Conjurer or a Thaumaturge could heal or do damage alone... but TOGETHER, they are the ultimate team.
As far as summoner -- and believe me, I'm looking forward to pet classes too -- perhaps that will be an 'advanced' class unlocked through one (or both!) of these classes. Summoners in FFXI were more or less the 'jack of all trades' in many situations. They could do damage, heal, buff, and enfeeble... They weren't the masters at anything, bu they could pull off most things... and that is where they came in useful. What if that class requires you to 'master' the entire discipline of magic before unlocking it?
I'm thinking other classes will branch off in a similar way. Will a Lancer become a Dragoon and a Samurai? What about a Ranger and Beast-master from archery? Will a combination of a melee class and a magic class mean you can play BLU? Only time will tell, but I think the most important thing to ponder is whether or not these 'advanced classes' will be unlocked and then count as a completely new class, or if they will be continuations of one class once you reach a certain level.
One thing's for certain: The best part about FFXI (compared to other MMO's) was the stories and the sort of identity you got because of it. With many different classes to choose from, you were KNOWN for your job (at least before everyone leveled 3+ 75's). Those 'moments' that were required to excel at that job really defined you and the people that went through them. Each combination of classes had a way of working together (remember when people specifically looked for certain classes to achieve certain skill-chains?)... And I hope that comes back.
As for FFXIV -- I love, love, LOVE the changes they've done thus far in functionality. It's more enjoyable to play... the only thing really missing besides a bit of sprucing up is the lack of special 'moments'. The 'community' feeling is already there.. now we just need the glue to keep it together. (I'm sorry for being THAT corny. It's 2AM where I am.)
And finally, I'd like to mention the fact that I think the classes were made in such a simple way at first is because they were rushing to get the game out. It was easier to shove all of these different abilities into one class (until 50, which probably won't remain the cap) than to separate them into many different ones when they had plenty of other things to worry about. I have a lot of confidence that these classes WILL split (or be added to) eventually... the only question is when.