So my sisters asked to do favorite pokemon teams. The results? Um....
Too many Eevees? No such thing it seems. Just keep it under half.
I think she is a bit attracted to derp-mons. ... and garbage.
In terms of the adventure only, what roles would need to be filled on a team? I mean roles such as: Fly, Surf, Fighter, Tank, Cleric, etc.
I personally don't see a need for fly. I would much rather travel on foot/bike around the countryside, taking in the landscape and the natural environment. It would definitely be helpful, but not necessary. Even something like surf could be questioned, as you likely could get to island cities by boat. It would still be needed for getting across rivers or to obscure locations, though, so I'd probably have a surfer on my team. Those are primarily navigation roles, and we can get around fairly well in our non-Pokemon world anyways, so while they would be helpful they aren't the most important. Fighter/Tank/battle roles are more subjective then anything else, depending on your goals in the adventure. If you are trying to collect the badges and compete in the Elite Four (or a tournament similar to the anime) you would probably care much more about team composition than someone who wanted to travel across the region simply for the experience of growing and surviving with your Pokemon. Cleric does seem important, though, as injuries will almost certainly be had and everyone will likely get tired.
I think some of the most important roles for Pokemon involve protection against weather or nature. If you travel somewhere cold, having a fire type Pokemon will be invaluable. The reverse is true if you are traveling somewhere really hot, and you need water to cool down in. A fire type (or something with a fire type move) can help navigate dark caves or at night, while something with defog or sunny day could help clear up unfavorable weather. While you would not necessarily need Pokemon with clear, competitive-based roles, you will likely still need something to help protect against wild, untamed Pokemon that could try to steal food sources or drive you away from nests you unwittingly stumble upon. You would need protection against rival trainers or thieves as well. This role could theoretically be filled by any Pokemon, however, as long as they were strong enough (which should come naturally over the course of the journey).
Besides that there are additional utility needs represented by the remaining HM's. Cut, Strength, Rock Smash can be used to clear paths or build shelters. Something with harvest could provide primitive food sources. A Pokemon with flash or a bright fire-type move to signal your location in case of an emergency. Something with electric powers to recharge the battery of a phone, pokegear, or pokedex (which will be important to keep functioning while far from society). I don't know how clean the water from water Pokemon is, or even if it's drinkable (I would be disgusted tbh), but if you desired you could keep something around to serve as a source of fresh water. A nocturnal Pokemon to keep you company while the others rest, or to keep lookout at night. A flying Pokemon to scout paths, look for help, etc. Something to possibly ride on if you get tired but can't stop for rest, or to help carry supplies. Something fast to run ahead or transport you quickly. I wouldn't mind something with dig, to create underground shelters which would feel safer than being exposed on the surface.
Really, there's so many different roles Pokemon could fill that it would be really hard to hit them all. I would just suggest selecting your favorites to travel with. But even then, in my opinion finding new ways to overcome obstacles with your favorite Pokemon is part of the fun and a crucial aspect of the experience.