These are just some ideas. I'll leave flexibility by giving a group to choose from instead of outright picking it for you ;)
1) Starter: Piplup, Chimchar or Turtwig. Honestly, I have no personal preference but I find Piplup line to be irresistible.
2) Normal Type: It can be anything that falls into the category so it can be the following Pokemon lines: Starly, Hoothoot Bidoof, Aipom, Buneary, Glameow, Happiny, Cleffa(Normal in Platinum), Munchlax, Lickitung, Togepi(Normal in Gen 4) and Porygon. Eevee too but I doubt you'll keep it as an Eevee. Personally I'd go with Buneary or Togepi out of respect for Dawn but I'll leave that to you. Normal Type really helps against Fantina and other Ghost Type users.
3) Ice Type: You'll likely want one due to Cynthia so it can be any of the following: Snover, Sneasel, Snorunt, Swinub and Glaceon. If you wanted to do a Dawn inspired run, Swinub would be my pick(plus it'd be solid against Volkner).
4) Bug Type: I find Bug Types to be really solid while also countering Gardenia, Lucian and anyone with Grass, Dark or Psychic Types. Plus Bug Types generally are ignored so I figure I'd suggest a run with one. Pick from Kricketot, Burmy, Wurmple, Heracross, Skorupi(even though it loses Bug Typing when it evolves, I'll still accept it), Yanma and Scyther.
5) Counter to Rival: Basically your rival takes the Pokemon that best defeats yours so you'll want something to easily defeat it. If you go with Piplup, then Bug, Fire and Ice work well. If you go Chimchar, then Electric would be my pick since Steel doesn't resist Electric. Finally if you go Turtwig, then Rock, Water or Flying(once it reaches its 2nd stage). Seeing as the choice here is dependent on #1, I'll leave this wide open.
6) Wildcard. Basically pick a Pokemon of your choosing that best fills out your team.