At some point several years ago, when I was a huge Undertale fanboy (I still kinda am, but to far smaller degree), I was thinking about how an Undertale Pokemon League would look like. It would... somewhat like this:

GYM LEADERS
1. Napstablook - Ghost type specialist (of course)
Their team would consist of Misdreavous and Phantump - two relatively harmless and friendly (if mischievous, in Misdreavous case) Ghost types. It also helps that Misdreavous was regularly portrayed in the anime as rather shy, fitting Blooky's personality. Also, Misdreavous would be their signature.
2. Toriel - Normal type specialist
Now, Normal may look like a weird type for her - most people give her Fire types. Thing is, said people make her Fire type specialist due to her being a pyromancer, but then give her Pokemon that don't really fit her in any way - simply because there aren't any Fire types suitable for her in terms of personality or apperance. So instead I decided to go the other way - first chose Pokemon, and then type.
Anyway, Toriel would have Chansey and Kangaskhan - two Pokemon known for their motherly, protective nature, with Kangaskhan as her signature.
3. Greater Dog - Ice type specialist
His team would consist of Snorunt (who resembles somewhat Ice Cap - an enemy found in the same area as Greater Dog) and his signature Snover (who - or rather whose evolution - similarly to Greater Dog himself is surprisingly friendly and curious despite scary looks).
4. Papyrus THE GREAT PAPYRUS, GROUND TYPE & SPAGHETTI COOKING SPECIALIST

• Marowak - his signature Pokemon and a rather obvious choice for a skeleton.
• Diglett - it being one of the very few permamently grounded Pokemon would allude rather nicely to Papyrus' gimmick of using telekinesis to ground the player during fight.
• Rhyhorn - don't know why. I just decided Rhyhorn would be good choice for him.

5. Mad Dummy - Electric type specialist (they use robots during their boss fight, so...)

• Rotom - Ghost/Electric type that often possesses electric appliances, and a signature Pokemon of a crazy ghost possessing a dummy.
• Magneton - robot-like Pokemon.
• Manectric - easily angered, teritorial Pokemon for easily angered, teritorial character.

6. Undyne - Fighting type specialist.
Once again, some would say that Water type would suit her better, but I think otherwise.

• Poliwrath - simply 'cause it's a Water/Fighting type.
• Primeape - always angry, never loses its target. Just like Undyne.
• Gallade
• Lucario - her signature. Both Gallade and Lucario are the "noble warrior-defender" types and thus would fit nicely with Undyne's "I CAN FEEL ALL THEIR HEARTS BEATING AS ONE" thingy.

7. Muffet - Spi... I mean, Bug type specialist. Obviously.

• Ariados - her signature.
• Galvantula
• Araquanid
• Masquerain - alright, not a spider, but its preevolution is based on a water strider. Which is ALSO not a spider, but it is an arachnid, so close enough.

8. Mettaton - Steel type specialist.

• Metagross - his signature, being a robot-like Pokemon and all.
• Mawile - Steel/Fairy type for the most fabulous robot out there.
• Doublade - Ghost/Steel type for a ghost-powered android.
• Bronzong - easily the most annoying Pokemon to deal with prior to Gen 6 for IMHO hardest boss in the whole game.

ELITE FOUR
1. Flowey - Grass type specialist.

• Ferrothorn - his signature. Just like Flowey himself, Ferrothorn fights using spiky vines - and it even resembles somewhat Flowey's Omega form!
• Sunflora - because it resembles him somewhat.
• Trevenant - becauseSpoiler: Major spoiler from his backstory

  He's actually Asriel resurrected as flower. Trevenant meanwhile - or rather its preevolution - is, according to Pokedex, a child resurrected as Pokemon.

• Carnivine - once again, alluding to Flowey's usage of vines and flytraps.
• Victreebell - ditto

2. Sans - Psychic type specialist.

• Wobbuffet - a lethal joke Pokemon for someone who prefers to catch his opponents off guard. His signature.
• Slowbro - there aren't any "lazy" Psychic types per say, but Slowbro is relatively close.
• Darmanitan - another gimmicky Pokemon.
• Xatu - it's future sight is sorta kinda similar to Sans' knowledge of alternate timelines.
• Musharna - dream Pokemon. Sans constantly dozes off. There's definitely some similarity.

3. Chara - Dark type specialist.

• Bisharp - their signature Pokemon, a blade handed monster for a child with... uh... rather worrying affinity for knives.
• Weavile - ditto
• Shiftry - ditto
• Crawdaunt - ditto
• Hydreigon - di... I mean, Hydreigon's destructive rampage, which ties nicely with Chara wanting revenge on humanity.

4. Asriel - Dragon type specialist.
All of his Pokemon are pseudo-legendary Dragon types.

• Dragonite - signature.
• Salamence
• Garchomp
• Hydreigon
• Goodra

CHAMPION - Asgore

• Slowking
• Seaking
• Kingdra
• Kingler
• Nidoking
• Nidoqueen

...Yeah, his whole gimmick is having a team of six Pokemon with "king/queen" in the name.