I really don't like this post. Many things are possible and work outside tradition and player base fixation and ideas of what a player should be. It's true that players think this way to cancel out your freedom, but things do work outside this phase and I'll give you some examples that are based upon but are effective and that probably 99% of players never thought of.
RDM/WAR as tank. This job combination requires a lot of player skill to perform effectively but I have personally done this in leveling parties. Equivalent damage taken to PLD/WAR previous to endgame gear and x2 MP pool for self curing along with bar(spell) makign it extremely effective against mob TP attacks that are elemental based and phalanx is extremely effective against very fast/weak hitting mobs. Postponed to endgame gear RDM tank is practical on magic damage dealing mobs.
RDM/NIN endgame tank is also viable. You can search this on Youtube for videos.
Tank class/BLU for super tanking. Combined with a THF or any class using SATA for boosted hate on the tank this is extremely effective for reducing damage taken beyond that of /WAR.
RDM/SCH for nuking. Combined with fast cast, convert, refresh, and B+ in nuking (/SCH allows this) makes RDM viable for DPS in this form. This setup is a viable replacement for both /blm and /whm. The only only advantage to either is elemental seal and erase at 75/37 (not including since Warp or Teleport since these aren't combat skills).
BLU/WAR for tank. In my opinion this the best possible setup for leveling as well as EG tanking especially with a genbu shield since you can stack cocoon and defender.
Now personally in FFXI I revolved around tanking/solo possibilities, but you think outside the box with a certain depth I'm sure this can be applied to other roles as well.
Not sure how well these would work out but some other ideas are:
1. BRD/COR or COR/BRD that doesn't pull. Maybe not the best for meripo but I'm sure this is effective everywhere else since it gives the job an extra buff.
2. BLU/THF for SATA enhanced damage. I have seen this before but BLU/NIN is very dominate compared to this which IMO dual wield can't compare to SATA and assuming you have a very good tank (which you should) then utsusemi is useless in parties.
3. Probably any DPS/THF for SATA + WS really.
4. DNC/NIN or NIN/DNC as tank is very effective assuming the player is skilled enough to build hate with these setups.
And I'm sure there are more setups I'm just not extremely familiar with DPS jobs.