I don't think it's a case of running out of ideas. I believe it's more a case of they've dumbed down and simplified so many things that they find themselves backed into a corner and have no room to expand or grow all those cool ideas simply wouldn't work because of how much they've already cut out and over simplified everything.
Every boss encounter in this game is essentially just a striking dummy with some heavily scripted dodge mechanics thrown in. Highly predictable and as such incredibly easy to handle. Bosses never adapt or react to what a party does they just continue to follow that exact same script every time.
Tanking has been trivialized to the point where it's press a cooldown every 45-60 seconds and that's it you've done your job... The reason why tanks focus so much on DPS is because what else are they going to do for the next 45 seconds??
Healing is basically the same. except in stead of pressing a cool down it's tossing a heal every 45-60 seconds.. and then they have nothing to do the rest of the time but dps...
DPS has been so heavily trivialized that is pretty much a preset rotation that never changes from one encounter to the next, hell if it wasn't for the inability to queue actions. people would quite easily macro there entire rotation and that macro would work in every fight...
What can the devs possibly do to expand this system and make it grow? Without some major changes not very much at all because they've backed them selves into a corner by dumbing everything down to a point where they cant possibly expand.
You could for example bring some classic FF status ailments into the picture. Slow paralyze, Blind, Oil. Confuse. and give jobs other areas to focus on other than dps..
However in the current state of the combat system that just wouldn't work. If a whm threw a paralysis on a boss and it procced on that bosses tank buster or next raid wide hit. you would quite literally break the entire fight because of it's highly scripted and predictable nature..
however if you were to randomize encounters you'd have room to incorporate these elements without breaking them. as paralyzing a tank buster would help but given that another one could technically come at any moment it wouldn't completely negate the risk of that damage. the same could be true of blind. if a big hit actually missed...
what you'd do here is create alternative ways in which players could interact with the battle and contribute to there party without arbitrarily defaulting to dps... if you randomized the encounters you'd also likely find healers healing more frequently because it wouldn't just be a case of 1 lustrate in 30 seconds is all I'm going to need...
Another option could be to rework positional bonuses so that they actually did something interesting and fun, Attack from the rear and break a bosses tail thus preventing him from using massive tail swipe cleaves, Or attack from a flank and you might wound a leg preventing dash mechanics from happening, Or break a weapon the boss is holding preventing them from shooting ranged attacks at people stood over there. Basically having fights that require different approaches and change according to your actions instead of just going into boss E and using the exact same rotation you just used in bosses A, B, C and D... but again this would require moving away from the tightly scripted and highly predictable encounters they've backed themselves into a corner with..
There's also jobs. and they pretty much are homogenized within there roles. i know some folks think the term is thrown around to much but the reality is they are all highly homogenized.
While it is true for example that a smn and blm may play slightly differently and both again play differently to a red mage. They are still homogenized classes. Each of them brings the same strengths and weaknesses to a party. its not as if a blm is weak at single target damage but excels at aoe and crowd control. where as smn may be weak at aoe, pretty good at crowd control while there egi rips single targets to shreds. they all have the same strengths and weaknesses to within a very small margin...
melee is the same you might expect a monk to excel at one on one but be lacking in aoe. where a dragoon may not be quite so strong or fast but with his longer range polearm might actually be able to crack several skulls at once,
same goes with tanks where they all share the same strengths and weakness again and again with healers...
every job is basically a jack of all trades master of none. and that pretty much makes them all homogenized because they all share the same strengths and weaknesses..
and thats another reason why they're backed into a corner and have no room to diversify or be creative..
look at tanks in 4.0 backed into the corner of being dps jobs the devs tried to get out of that corner said hey guys defense matters took away strength and tried to entice players with tenacity. and players just laughed at them and kept using 270 accessories pushing tanks right back into that corner..