There are a few ways to add mouthfeel to mead.
1. Add more honey - Even if it ferments dry, more honey always gives more mouthfeel. If it doesn't ferment (sweet mead), it will have a lot more!
2. Tannin - This is what you were getting at with adding raisins, but you would have to add a LOT of raisins. Better options are grape tannin or oak cubes. You can purchase them from online shops such as morebeer.com.
3. Glycerin - A wine makers trick for adding silkiness to big reds. Make sure the yeast are maxed out on ABV or stabilized as yeast can metabolize glycerin.
4. Spices - Some spices add a perceived mouthfeel. Gesho immediately comes to mind. Coriander, Szechuan berries, and black pepper also add a bit of mouthfeel if used sparingly.
Better brewing through science!