The main thing is you'll have to play through the MSQ (Main Scenario Quests), which has EVERYTHING locked behind it. Having a private room in your Free Company's house is locked behind it at 50, for example. Retainers (bank equivalent, plus auction house listing abilities) are locked behind it, albeit low. Getting your mount is locked behind it at 30. Training your mount to fight as a companion as well.
As for transmog (it's glamour here in Eorzea), that would fully depend on what set you like whether you'd need to get into Heavensward content, of course. Again, trust me, I've been an alt-player in WoW and even to some extent in EverQuest a little bit. I know how where the road leads.
"I just need 50 to get the gear set I want and then I can just log that character in for RP when it strikes me"
*next patch*
"O_O Dat....dat....dat mount is PERFECT.... I need to level to 60 and get the gear to get it."
So the time commitment does grow. I also have a nutjob friend who doesn't RP, but has character personas in mind for his alts and he keeps 3 at max level and geared in two sets (tanking + one other). He's a special bird. >_>
Sorrior's solution isn't bad, though. Make a character in every slot, mass transfer at once for $20 and use fantasia later if you want to change races/appearance (they will each get 1 free fantasia). Name change is $10, I think, if that is required down the line and isn't part of the Fantasia.
Not the best work around, but it at least gets everyone on Balmung!
If you downgrade to 1, I think it's the first one on the list, but I could be wrong.
However, people tend to overlook this:
1 character sub = $10/month
8 character sub = $15/month
8 character sub, 6 month payment = $60/twice a year
So, in other words, the 1 character/month cost can be matched if you're okay paying for 6 months of game time at a time. I use this one. Even if I take a break and don't play a couple weeks (Star Fox 0 and Uncharted 4 just came out), I'm still getting time towards my veteran's rewards. *snicker*