If i got your point right.. you want conditional requred fields... 
For example when some field changes depending on value of that field other field is required, or hidden ... or something.. 
Yes, you can do that, trough view definition (example);

<field name="some_bool">
<field name="some_selection">
<field name="some_value" attrs="{'invisible':[('some_bool','=',True)], 'required':[('some_selection','=','some_sel_value)]} />

In this example view def you have conditional visibility, and conditional required field... no on_change needed 
and if needed complex required check... you can write some method that returns True or False and call it instead of putting [()] conditions in brackets...

Hope this helps..