Collaborating on projects should work well using SVN - see the how to collaborate on projects with SVN tutorial. SVN is one of my favourite pieces of technology and I think it does what it does extremely well. In fact, IMO it's almost unbeatable. So I really don't want to reinvent the wheel and try to duplicate anything it does in C2 itself. SVN is also an industry standard technology, so you're learning a useful tool that can be used in many other places in the industry. The same tool can be used to collaborate on documents, animations, Spriter projects, javascript code (e.g. using the SDK), and so on. If we built anything like that in to C2 itself, you wouldn't be able to use it elsewhere in that way. So I think it's actually better to leave the job to SVN.
You can attach a .capx file to tutorials in the tutorial system, and I think that works better than having a forum: the presentation is better, it's categorised and more searchable, has ratings and rankings, translations, and more. So I think if you're looking for guides and examples, the tutorials section does that already. If you think it could be improved, we're happy to take suggestions - perhaps we could add an 'Examples' category dedicated for basically attaching .capx files?
As the future plans blog post stated, the main reason it's not already in the works is it's a very complicated feature and will probably take some time as we rearchitect different parts of the engine. This also entails engineering risk: there could be significant delays while we work out problems with the rearchitecting, or if something goes wrong, we may need to backtrack. We have a few other projects on the go in the mean time which are hopefully going to produce great results with much less time investment/engineering risk. I think that is the best order to do things in; engineering risk is an important thing to take in to account, but I can imagine from the outside it might be easy to disregard. Anyway, we know modularity is important and we will get to it eventually, it's only a question of when. (I'd point out we've also run feature request polls in the past where other features have topped modularity in votes.)
An asset store is something we've discussed internally before. There are some complications, particularly with legality/accounting when allowing effectively anyone to become a seller via us. I can also see quality assurance also being an issue (we should prevent anyone drawing some scribbles and trying to sell them for $10 - if we end up full of that type of thing it drags down the utility of the whole store). However if we did add modularity features, a place on Scirra.com to at least find pre-built modules is something we'd be very keen to add.
Unity is a radically different tool to Construct 2. Just for starters, it's heavily coding based, and at the moment doesn't really do much in the way of HTML5. If you decide Unity is the appropriate tool for the job, then you must be after something significantly different, so go for it! Hopefully Construct 2 will still serve nicely when you have something you want to develop quickly, or web-based, or without coding, or using tilemaps, or whatever else - don't forget C2 has its own strong points too :)