Force a brainstorm into real, differentiated options instead of one idea repeated five ways

Generate exactly 5 distinct ideas for the brief below. "Distinct" is defined precisely: each idea must differ from every other one on at least one of the axes under DIVERSITY REQUIREMENT. A rewording of the same core mechanism does not count as a second idea and will be rejected — check this yourself before presenting the list, not after I point it out.

BRIEF
Ways to get more of our existing email subscribers to actually open a Tuesday product-update newsletter, currently at a 12% open rate.

SUCCESS METRIC
Open rate on the Tuesday send, currently 12%, measured over the next 4 sends. Every idea should be evaluated, implicitly, against whether it plausibly moves this metric — an idea that's clever but disconnected from what actually matters here should be cut before it reaches the list, not included for variety's sake.

DIVERSITY REQUIREMENT
Across the 5 ideas, cover at least these different approaches, one idea per approach where the brief allows it:
- Change to subject line or framing
- Change to send time or day
- Change to content format, not just wording
- Change to audience segmentation
- A structural change outside the email itself (different channel or cadence)

HARD CONSTRAINTS — KILL CRITERIA
No budget for new tooling. Must be implementable by one person in under a week. Cannot require re-platforming the email tool. Cannot reduce send frequency below once a week (contractual with sponsors). Any idea that violates one of these does not make the list at all, no matter how good it otherwise is — do not include it with a caveat that it "would need approval" or similar; if it fails a hard constraint, it is out.

FOR EACH SURVIVING IDEA, STATE
1. The idea itself, in one or two sentences — concrete enough that someone could start building it tomorrow, not a direction or theme.
2. Which diversity axis it represents.
3. The one real tradeoff it makes, named specifically — not "requires more effort," but the actual thing given up (reach, speed, personalization, control) to gain the thing this idea is betting on. An idea presented with no real cost has not been thought through.

IF AN IDEA SPANS MORE THAN ONE AXIS
Pick the axis it represents most strongly and assign it there — do not let one idea count toward satisfying two axes at once just to shorten the list; the diversity requirement exists to force real spread across approaches, not to be satisfied on a technicality.
4. A rough size of the tradeoff, in the same units as the success metric where possible — if the metric is open rate, say roughly how much of a hit to some other number (send frequency, list size reached) this idea trades for it.

AFTER THE LIST
Name which single idea you would personally bet on and why, in two or three sentences — an actual opinion with a reason, not "it depends on your priorities." Then name the one idea on the list you'd cut first if forced to ship only three, and why — a ranked list, even an informal one, is more useful than a flat set with no ordering signal at all.