Mark/Dash is almost balanced
Only 2 things remain and this will be a healthy summoner spell for ARAM. In fact, if you fix the following tools you can probably even bump the CD down to 30 seconds and you still won't really see long range carries taking the summoner anymore.
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Minion Dashing. This is a counterplay issue. If I see snowballs coming and I take the effort to dodge it, I should be able to successfully avoid the engage. Minions shouldn't be taggable by mark. This increases players ability to aim the snowball while removing the BS aspect of dashing to a minion line with Galio and getting a free 5 man ultimate.
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Untargetability/invulnerability while dashing. Just remove it. Allow people to be CCed out of their dash and allow them to take damage. People are using this to dodge skillshots amongst other things and it's just a silly mechanic to have. It's a huge reason why snowball is still viable on ranged champions that clearly would want to avoid gap closing.
Side note: You really need to nerf Ludens Echo on ARAM. Make the AOE prioritize minions unless the skillshot has struck a champion. Alternatively, remove it altogether and replace it with something else. It lacks counterplay and gives big AP damage to champions who really don't need it (Ex: Janna/Nami/Sona/KogMaw)