play something inventive and focus a class towards something other than what it has been doing for the last 9 years...
Possible, but as some have nicely pointed out, versatility =/= utility.
As a DRK main who has mainly played WAR or BLM since returning from a break (cuz thats whats needed) I still work on my DRK...so heres the basics.
Melee jobs benefit from melee subs, depending on level you either get ACC bonus (DNC DRG RNG) lolEVA, Haste, Dualwield, or TP gain bonus, one job gives all of those.... /dnc.
Some jobs give ATK bonus but its gimped (or is same level DRK gets already) minor stat bonus are also rec'd from various jobs.
/NIN = hate you wont pile on Mage (+Survivability) /SAM = mainly TP gain bonus with a little survivability
/DRG = Haste (w earring) ACC bonus, jump.--- None of these jobs can: curaga when party is slept (/DNC can) or Silena mage (/DNC can) return MP for no casting time (/DNC again) return HP (/DNC) Free Sneak/Invis. (/DNC)
/DNC can also Debuff w steps (thus building up moves for Voke or Grav.) or Store TP for mini Sekkanoki.
I have had great fun DRK/SCH when I was doing campaign....also its a monster for Campaign EXP & Drops. (can cap all categories)
/RNG is a beast low to mid lvl soloing/duoing (with bolts) + widescan for NM's & such.
That being said, play how you want but it may severely limit you, remember... everytime you cast /mage, your NOT swinging your weapon, unless DRK gets some fast/insta cast this will always hurt.
(currently I'm working on DRK/MNK & it is insanely good this is however : at 90, in Abyssea, with proper Atma & Abyssite though)
BASTOK:10 SKY: O SEA: O DYNAMIS: O