Are there any restrictions on which houses can be combined in a deck in Keyforge?
No. Not only is no combination blocked, each combination is equally likely.

Here are the number of decks registered on the Keyforge Compendium for each combination of houses:

: 23,360 (3.1%)
: 23,363 (3.1%)
: 23,435 (2.8%)
: 23,454 (2.7%)
: 23,606 (2.1%)
: 23,607 (2.1%)
: 23,636 (2.0%)
: 23,639 (2.0%)
: 23,723 (1.6%)
: 23,729 (1.6%)
: 23,810 (1.2%)
: 23,818 (1.2%)
: 23,857 (1.0%)
: 23,876 (1.0%)
: 24,053 (0.2%)
: 24,230 (0.5%)
: 24,265 (0.6%)
: 24,280 (0.7%)
: 24,294 (0.8%)
: 24,318 (0.9%)
: 24,330 (0.9%)
: 24,350 (1.0%)
: 24,359 (1.0%)
: 24,386 (1.1%)
: 24,397 (1.2%)
: 24,408 (1.2%)
: 24,420 (1.3%)
: 24,455 (1.4%)
: 24,488 (1.6%)
: 24,512 (1.7%)
: 24,583 (2.0%)
: 24,625 (2.1%)
: 24,700 (2.4%)
: 24,733 (2.6%)
: 24,759 (2.7%)

(The percentage is the difference from the average, as a percentage of the average.)