I was thinking about this yesterday when picking army lists. Much as I love the Space Marine Codex there's no denying that it lacks a certain something compared to it's more recent bretheren. However, As opposed to a complete rethink I'm more of the opinion that a bit of tweaking here and there would be more than sufficient to bring it up to speed (and up to date). Here's a unit-by-unit analysis of what I think:
Special characters - as for now except with an Iron hands character added in. Each chapter specific model may take their own chapter tactics at no extra cost.
Chapter Master - add more exotic wargear options to differentiate from captain.
Honour guard - add the options for combat/storm shields
Captain - same as now
Command Squad - include the option for jump packs
Librarian - add in a new set of 6 (7) SM librarian powers as some are repeated in the new generic powers.
Chaplain - no changes needed
MoTF - add in more archeotech wargear
Add in a space marine specific chart for chapter tactics (a similar chart as for warlord traits, but that works in addition to). These come specified for special characters from the appropriate codex. If a generic character is taken, they may choose to roll on the table (no cost) or may select a specific chapter tactic for a cost specified:
1 - Ultramarines - combat tactics (free)
2 - Imperial Fists - stubborn
3 - Raven Guard - fleet?
4 - Iron Hands - army gains a toughness bonus?
5 - Salamanders - twin linked flamers/meltas?
6 - Whitescars - outflank?
Tactical squad - as now, but with more sergeant options and can take special weapon with 5 min models not 10.
Scouts - as now, but add in land speeder storm as dedicated transport
Rhino - no change
Razorback - make heavy flamer free swapout for TLHBs
Drop pod - now only 10-man capacity
Land speeder storm - now a DT
Apothecaries - make available in same way as sanguinary priests except only affects squad, not area affect. (models 40pts basic)
Terminators - add in combi weapon options for sergeant
Assault terminators - makeTH/SS a 5pt upgrade per model.
Dreadnought - same
Venerable dreadnought - same
Ironclad dreadnought - same
Sternguard - same
Legion of the damned -
Techmarine - either as now or as an option that doesn't take up a slot (same as SW codex)
Bike squad - as now
Attack bike squad - as now
Land speeder - as now
Stormtalon - add in
scout bikes - same as now
Assault squad - add in options to remove JPs for a DT discount and take melta guns
Vanguard squad - cheaper base cost off 115, otherwise same
Support weapon squad - any combo of thunderfires and rapiers up to 3 guns. Rapiers 2 marine crew, thunderfires 1 techmarine.
Land raiders - all same except normal LR has capacity of 10 models.
Predator - same
Vindicator - same
Whirlwind - option for hunter AA missile
Devastators - points costs in line with BA codex, so 10 for ML/HB/MM, 15 for PC, 25 for LC.
Lower costs of storm bolters on sergeants in line with BA codex
Increase costs of storm shields in line with BA codex
Add flakk missiles to missile launchers.
Well that's my thoughts on the topic - please feel free to chip in others or pass comment on the above points. Cheers.