Yes, you can cosplay a character that isn't a hero at Heroes Dutch Comic Con. The event generally allows any fictional character as long as your costume follows their safety and decency guidelines. To be fully sure, check the official website for the most current rules on weapons, props, and nudity.
The convention's name might suggest a focus on heroes, but in practice it welcomes all genres: villains, sidekicks, original characters, anime, video games, movie monsters, and more. The key rules you need to watch are about weapons and prop policy. Any realistic-looking guns, knives, or sharp objects must be clearly fake, non-functional, and peace bound (secured so they can't be drawn). Some props like metal swords or bows with real tension may be banned. Also avoid anything that could be seen as offensive or hateful. Common sense dress codes apply: no full nudity, and any suggestive costumes should still be family friendly.
Your next step is to visit the Heroes Dutch Comic Con website and look for their "Cosplay Guidelines" or "FAQ" section. They update rules per event. If you still have doubts, you can email the organizers or ask on their Facebook page. You can also check Facebook groups like "Dutch Cosplay Community" where experienced attendees often share their experiences. Since you have 3-4 months, you have plenty of time to plan. Start your costume early so you can adjust if needed. Remember that prop restrictions are the most common reason for cosplay being denied entry, not the character itself.
To summarize: pick the character you love, make or buy the costume, but confirm the prop rules before the convention. If your costume includes large wings, height extensions, or special effects like smoke, check if those need prior approval. Enjoy the con and don't worry about character choice.