For starters, I like this:
Add to that the following:
A new enhancement line...
Monk Strike (Stunning Blow) I: +1 to stunning blow and stunning fist DC. AP Cost: 1, Level: 1, Progression: 0, Requires one of: Stunning Blow or Stunning Fist
Monk Strike (Stunning Blow) II: +1 to stunning blow and stunning fist DC, bringing the total increase to 2. AP Cost: 2, Level: 6, Progression: 18, Requires: Monk Strike (Stunning Blow) I
Monk Strike (Stunning Blow) III: +1 to stunning blow and stunning fist DC, bringing the total increase to 3. AP Cost: 3, Level: 12, Progression: 41, Requires: Monk Strike (Stunning Blow) II
Monk Strike (Stunning Blow) IV: +1 to stunning blow and stunning fist DC, bringing the total increase to 4. AP Cost: 4, Level: 18, Progression: 64, Requires: Monk Strike (Stunning Blow) III
...and consolidate elemental stances...
Rebalance elemental stances so that there are only 3 tiers. Tier 1 at first level for free as it is now, Tier 2 at 9th level for AP cost: 2, then Tier 3 at 18th level for AP cost: 4. The current Tier 4 would be exactly the same as the proposed Tier 3. The middle tier would need some readjustment, but each tier should get an ability bump (i.e. Tier 1 gives +2, Tier 2 gives +3, and Tier 3 gives +4).
Overall this may improve stunning DCs a little if the monk stays pure and invests points into the DC line, and consolidating the elemental stances to 3 levels at the proposed costs will help free up some action points to spread around. The levels for Tier 2 and 3 could even occur slightly earlier (ex. level 8 and 16 perhaps) so as not to discourage moderately/heavily cross-classed builds.