When America's Test Kitchen was making grilled cheese, they mentioned some problems with aged chedda ... namely that it's dry and doesn't melt well. They got around this by adding in a cheese that melts well, plus extra moisture. Their cheese mixture was:
7 oz aged cheddar
2 oz brie (rind removed)
2 TB dry white wine
2 tsp minced shallot
They also used mustard (but on the outside, not inside).
You might not have the same problems, as you'll be adding milk or other liquid to change the consistency.
Personally, if it were me ... I'd look to two potential dishes as a starting point : Welsh Rarebit (eg, see Alton Brown's take on it or fondue (there are lots of cheddar-based adaptions out there).
One common thing in all three of the things I've mentioned is that they all use alcohol (wine, beer, brandy, hard cider, etc.) as extra moisture rather than something heavy like milk, and some extra flavoring (shallots, mustard, garlic, etc.)