Would like to suggest a separate "Comment" application instead of the standard comment form. The app should have a customizable input fields, other than the comment field.

From what I understand, you're asking for something similar to IP.Content?

I'm not really sure but I'll give an example usage.

Scenario 1: Computer Hardware Forum

1. Admin create a forum.
2. Admin create a new "comment form" in the Comment Application. The custom fields are CPU, Motherboard, Memory, etc.
3. Bind the newly created "comment form" to the created forum in Step 1.
4. When a user replies to topics in that forum, he needs to fill-up the standard comment field, CPU, Memory, etc.
5. "comment form" can be bind to more than one forum.

Scenario 2: IP.Content Hotels Database

1. Admin create a new instance of "comment form" for this hotels database with custom fields such as Your Rating, Pros, Cons, Conclusion, etc.
2. Bind this "comment form" to this hotels database.
3. When a user replies to an entry in this database, he'll fill up the Ratings (can be dropdown from 1-10), Pros, Cons, etc.

Scenario 3: IPS Community Suite Support Forum

Same as #1 above except that the custom fields in the "comment form" are PHP Version, MySQL Version, IP.Board Version, Installed Apps, etc.