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There are several mono-type teams that I wouldn't mind having if I was an Elite Four member (because frankly unless you're the Champion you're generally monotype or if you prefer, themed). Chances are though, I would most likely decide upon being a Water Elite Four member.
Rotom-W
Starmie
Jellicent
Milotic
Kingdra
Gliscor (Trump Card, so to say)
Probably swap a few Pokemon around and such, but this is the general idea, so to speak.
Hmm.
If I ran a monotype, it'd either be water (in honor of my Rain Dance team) or Bug (Only been done once.)
Water:
Swampert
Kabutops
Starmie
Kingdra (Key)
Jellicent
Vaporeon