Do you folks actually want Marksmen to be optional?

If yes, are you willing to see Rift take massive, systemic changes to the way XP generation and Tower pushing work?

Because, ultimately, that's what would be required.

As it stands, Marksmen are needed for two major (and one minor) reasons.

1. Ranged damage against Towers is absolutely required to take them when they're defended. (major)
2. The duo lane needs a champion in it that scales well with gold, but still functions well with a level disadvantage against solo lanes. (major)
3. A source of high Physical damage is required to prevent the enemy team from just stacking MR. (minor)

Solving problem 1 either requires some kind of champion specific solution (see Mord) or a complete rework of how Towers are destroyed that makes it transparent to what champion you're using. Probably the simplest solution would be to channel damage at range ala Dominion. A value that scales with level that would allow all champions to push towers with equal safety and effectiveness.

Solving problem 2 would basically mean champions getting full XP on minion death, whether that be a melee champion specific thing or a map-wide thing.

Problem 3 already isn't that huge a deal. Just make sure you have some combination of two AD Fighters/Assassins/Juggies somewhere on the map.