Ideas:
Eating bars, an upgrade that you can buy to make the bar extend slightly outwards and act as a table
Increase tavern size upgrade, start with a smaller tavern
Some people will start fights, kick them out yourself, hire a bouncer or buy a fight pit
More food, whisky, mutton, breadsticks, water
More classes and races, cleric, bard, rogue, orc, goblin, ghost
More furniture, bar stool, menu, painting, long table (Takes up two squares), cloth chair to match cloth table
Prince/Princess, appears five days before the king with regular customers, will judge you on how well you serve them and others, fail and the king will not appear until the prince/princess returns and finds your tavern acceptable