Every tank have their basic Enmity skills, so Enmity is not a problem.... i mean look at RDM they have their both Veraero and Verthunder (300 potency) at low level ! and i never had a problem with Enmity, but lets belive im doing extreme Dmg and my tank cant keep agro:
a) in low level Dungeons i wont die from getting the aggro, honestly a BLM can tank in there, specially bosses.
b) i have lucid Dreams and such to reduce enmity, wich, YOU being a lvl 70 know what to do to decreast Enmity so im pretty sure you know how to deal with that in case you dont want to tank (wich you totally can)
c)Ultimately just buff enmity generation from Tank's Basic combo, or make an Echo like buff to tanks to increast their Enmity Generation.
Now for the other problems addressed:
1) Healers: honestly, healers have 1 heal skill at low lvl wich is not difference from healing the tank or healing a dps.... like it happens all the time they just need to heal the person taking dmg, any new healer can do that (about the not having ress yet, im sorry but if you need ress in a low level dungeon as a lvl 70, the problem is yours)
2)Being unfair for new people doing the dungeon for the first time: Yeah totally spamming 2 skills is an experience no1 wants to miss, almost EVERYONE at some point complained about how boring level (specially for some classes) is on this game, Thats why you do potd anyway, cuz having 1 skills is boring as hell, Guess what, when you were new you thought that way, so yeah, those who are now new, also think that way, lets say that an THM lvl 20 gets matched with a BLM lvl 70, The sync being only stat wise so the BLM have its full end game rotation..... Wouldl the THM feel "omg he is doing it better than me cuz those skill, i suck, my life suck, this is so unfair i will quit the game for the unfairness" or would it be " OMFG ! LOOK AT THAT EXPLOOOOOOOOOOSION OMG ! AND THAT ANIMATION OF A CIRCLE ON THER FEETS OMG HE IS DOING SO MANY COOL AF MOVEMENTS I CANT WAIT TO GET TO 70 AND DO THAT OMG ! " ... Just think about a summoner who just got his ifrit, would he be overwelmed and feel bad about the other lvl 70 summoner with all his abilities doing too much, or would he be "OMFG THATS A FREAKING DRAGON ! I WANNA HAVE A FREAKING DRAGON TOO IM GONNA HAVE A FREAKING DRAGON I CANT WANT! "
.... Point made, if anything it would be an incentive for new players to look at the cool stuffs they could do if they make it to max lvl.