Online modes and features and execution of Career Mode of Forza Motorsport 3 are definitely NOT something that would go into some "perfect racing game".
Online modes are far inferior to Forza Motorsport 2, which is the good model but it lacks qualifying. So, in a search for perfect online in console games you should refer to Simbin's Race Pro on X360.
As for Career mode, "perfect" one would probably be some crazy mix of GT2, Race Driver 3, GRID, GT4, Forza 3 and GT5. Forza 3's career is almost pointless as GT5, altgough it have much more actual content. However, actual execution and lack of focus after you reach Level 50 - which I've reached with less than 20% completion of Event Mode - is just opposite side of same crazy logic as reaching Lv40 in GT5.
Also, in catering some *perfect* console racer, you're missing many games that have great unique features which could/should/have been incorporated into some *perfect* game.
From my head and with no particular order those are:
> LeMans 24 Hours (Eutechnyx) - real-time engine adjustment
> Race Driver 3 (Codemasters) - sheer amount of licensed real-life circuits and content, progressive mechanical damage system
> F355 Challenge (AM2) - Great Driver Challenges, vehicle focus, post-race analysis, Magic Weather option
> Race Pro (Simbin) - online options
> Project Gotham Racing series (Bizzare) - real-life cities/real-time weather system/almost perfect cockpit position/full 3D garages and FP free-walk
> GRID - visual vehicle damage system
> WRC 4 - rally tracks and everything rally related
> F1 Championship Edition - in-race save
etc, etc, etc
Forza 3 and GT5 combined could be a great game, but *perfect*... not.