I have very little experience in designing HASL's (only advising/ co-authoring on two projects so far.)
"I'm curious what elements in a battle designers look for when selecting a subject for a HASL. (HASLing? HASLification? HASLization? HASLed?) "
From that limited perch, this is my view:
1. It needs a "hook" - something to catch the players eyes and interest. Stalingrad does it by way of being so well published in history books and examined from almost every angle in wargames. Ruw Ridge does it with the total DTO Cptr F being used hook, Gavutu-Tanambogo- Seaborne assaults and total PTO as a hook. Cem Hill, a full on airborne assault, and so on (just mentioning a few prominent examples). As a designer, I need an area that has enough of a variation to capture players interest enough to have them want to try it.
2. A variation of battles occurring over the same, or similar, terrain. Attacks, counterattacks, night assaults, rain or mud or snow in play, mist like KGP in play, and so on. There needs to not only be enough variation for the CG(s) on the historic map(s); there must also be enough to include a showcase of 6-10 scenarios that help players explore any new rules that come with the HASL. (I personally prefer the idea of say 4-5 sens on the HASL map and 4-5 more on geo boards - going the HSASL approach route, myself.)
3. It needs to be contained as a battlefield. Doing all of Kursk in a single HASL is impossible. Doing sections of it, however, is quite feasible.
4. There need to be some new rules sections, but not too many. More than 3-8 pages of new rules are going to make it hard to get players to bite and try it out.
5. it needs to be something not overcooked in ASL. We don't need ANOTHER stalingrad HASL, (well, I do, but that's because HALS CGs are my own niche of enjoyment in this hobby), and so on. Bring something new to the table.
6. It needs to be timely. ABtF has been published for ages, so let's not do one now on say, the LZ fighting that the 1st Airlanding Brigade did on days 1-4 of Market-Garden, and so on. (Regardless of whether or not is "new" in the sense of a HASL CG and map designed around farm fields and sparse wooded areas with irrigation ditches.)
That's about all I got as a (quite limited) designer for HASL stuff. When I found these kinds of things all present in one package (twice now), I got with a few other like-minded ASL designers and we got busy working on those ideas to see what could become of it.
KRL, jon H