You can have pretty interesting Monopoly games. Mine usually end up with interplayer corporations, loans for building and investing, bailouts, mergers and similar.
At this point you aren't really playing Monopoly anymore. You've developed your own game using the Monopoly pieces.
As long as everyone plays by the game rules, any out-of-game interactions (such as agreeing to pool money and property) don't mean you're not playing monopoly any more.