I'm glad you brought up Mark/Dash because I really think Riot's making a huge mistake by adding it for two reasons:
Lack of engage is rarely directly the deciding problem against poke comps. The problem is that you'll end up either losing the engage or you win the engage and it's not worth it. There are a lot of ways that winning an engage can make it not worth it. For one, melee comps (the ones that need to hard engage) have an exceptionally difficult time pushing turrets, so unless you get a clean ace with a minion wave coming then you're not going to get a turret out of it. Another case is where you trade somewhat closely. In that case the poke comp ends up in the better position because they can just continue to poke.
Keep in mind there are already a lot of tools to engage with. Melee champions tend to already have plenty of engage tools in their kits in the first place. On top of that, there are plenty of items like Righteous Glory and Talisman of Ascension that make engaging pretty easy. Of course there are certain comps that are still very difficult to engage, but they're again so rare that it's really not worth balancing around.
Lack of engage is usually just one several other problems that come together to make certain comps have a hard time. It's more the conglomeration of lack of sustain, lack of waveclear, and lack of income that cause the problem. Once you get behind because of those factors do you start to feel like you can't engage. Adding Mark/Dash is attempting to fix a symptom, not a cause.
The second reason I think Mark/Dash is a huge mistake is because it's available for everyone. There are already a lot of extremely strong champions that would benefit much more from a strong engage tool like Mark/Dash. The ones I usually site are Galio, Amumu, and Alistar. Having a low risk, low cooldown engage spell on those kinds of champions is going to cause more problems.