Too bad the other forms of Rotom are not available yet... :/
Luxray and the ep.10 Elekid are good choices... But, if you joined the Magma gang, you might have a Chinchou in your box? That could be somewhat useful (If you don't have it, well... sorry...) If you're a fan of Pikachu and have enough pacience to raise a Pichu, you could also use that... Joltik is around dem parts also... And if you have a Growlithe, you could potentially have an Ampharos...
For a grass type, Venusaur is probably the best one... You're going to raise a Chmecho first though, otherwise no Venusaur (See the Pokemon Location Guide for more details...) Maybe Ludicolo also? It doesn't have a weakness to electric or ice... That's useful...
You have a Gyarados? MOTHERFUCKING GYARADOS? The star of my current team? The destroyer of everything that doesn't have an electric attack? Your team needs nothing if you have a Gyarados (Maybe a Ground type...)