New Ideas 

• more ways to increase stats. by doing challenges/courses/achievement's at the moment seems to take forever to increase stats such as marksmanship
• crew properties/business - nightclubs, casinos, mansions  which give you certain benefits. crew members work together to buy properties protect them from raids and so on..
• crew territory/wars - be able to attack, mug or leave rivals  allowing you to gain more experience for levelling   and gain respect for your crew.
• marriage - be able to marry - joint bank accounts share armoury, properties buy gifts.

These all seem like good additions, there will need to be some narrowing down and actually expanding upon ideas but overall I like it

• more ways to increase stats. by doing challenges/courses/achievement's at the moment seems to take forever to increase stats such as marksmanship

Your stats are already increased by the activities you do in the game such as jails, petties, felonies, wacking, PPing, etc... You can also boost your stats using hustle points. Achievements give you XP and/or cash and/or credits as is, what more do you want for free?

• crew properties/business - nightclubs, casinos, mansions  which give you certain benefits. crew members work together to buy properties protect them from raids and so on..

Casinos already exist. This game is an RP game, most of these businesses you have suggested exist in the Business Districts in the form of roleplay threads.

• crew territory/wars - be able to attack, mug or leave rivals  allowing you to gain more experience for levelling   and gain respect for your crew.

This is what districts are. You can PP anyone anywhere, you just take the risk of them mugging you or PPing you into your own hands when you do so. You get XP for this as well.

• marriage - be able to marry - joint bank accounts share armoury, properties buy gifts.

Again, this is a role-playing game, roleplay your marriage as it has been done more times than I can count in the past. Also, how would you address the issue of joint accounts and armories, properties if the spouses are in different districts or cities? You can buy perks as gifts for everyone and anyone already, are you proposing adding trinkets and jewelry into the mix too?

This is a role-playing game and every single one of your suggestions takes away from the role-playing aspect of the game in my opinion save for the first that is redundant to the already ample ways of increasing your stats in place already.