There's some additional late game companions in act 2 and 3 but not too much. A lot of casters (including druids) will also gain elemental summons as spells in later levels. The summon itself can be a tank for you. You can also respec characters at Withers in your camp. You want to be careful changing origin characters classes, but subclasses are fine. Clerics can be great tanks, especially in the life domain. For example my current character is a cleric/monk with an armor class of 24, almost ever attack misses them.

EDIT: removed my first sentence, it mentioned the class of a later game companion which may be considered a spoiler to some.