I'd sortof like to change the if plugin config to bring the expressions to the value side of a dict, so that we can validate it with a schema. We could add a custom schema 'format' for these expressions to catch syntax errors during validation. Not exactly sure what the nicest way to do that is, and if it's worth the extra validation ability though.
perhaps:

if:
- if: year >= now.year
  action: accept

It sorta doubles up the 'if's, but it might also open room for an 'else':

if:
- if: year >= now.year
  action: accept
- elif: year < now.year - 2:
  action: reject
- else:
  action:
    imdb:
       min_score: 6.0

Actually, I got stuck on the else bit. Is the implicit 'None' there horrible? I sorta think it is.

I think the implicit None is fine, if plugin was always programmatic in nature, this fits in well.
schema format is a good idea but since a lot of fields in flexget use jinja not sure if it's relevant only to this plugin.

Awesome! I always told you eval is evil! :)

I think we'll just go with the engine change for now. Config format can be revisited later.