I think this is the most important aspect here. It has to qualify as 'true' Jagged Alliance for long-time fans, but also needs to be mainstream enough be enjoyable for as many non-hardcore players as possible. Developing games isn't as easy or cheap as it was some 20 years ago, so for the sake of commercial success some concessions are necessary.
While I noticed that some people here don't like the new XCOM, they probably don't know that it may be the only reason we get to see JA3 in the first place, as it did nothing short of a small miracle: XCOM revived a dead genre and was a commercial success. It's only natural that any investor would want JA3 to be as similar to XCOM as possible to minimize financial risks.
And that's what we can see from the few gameplay scenes we were shown: the cover system, skills, even the overwatch symbol are close to XCOM. That is also why we won't get 1.13 2.0.
That being said, I'm quite hopeful that we're on the right track this time around. I don't see any hints that there's a restrictive class system with odd weapon restrictions, I can see that there's weather, there's some inventory management, you have more than one squad, you have three stances (likely standing, crouching and lying), firing modes for weapons, body part aiming, an AP based combat system... all in all more than XCOM had to offer here.
Combat-wise I'd say that JA3 already is the 'true' JA experience. Although that isn't necessarily a good thing. The biggest challenge for Firaxis was the flow of combat. XCOM 1 was - not unlike JA2 - a slugfest of crawling, camping and waiting for interrupts/overwatch. That's the reason for those time limits in XCOM2. I can't deny that the flow of combat has indeed gotten better, despite hating the time limits. I think this is also the biggest challenge for JA3: to make combat faster/discourage boring overwatch-camping without imposing severe time limits or punishing players for not overwatching everything to death.
That leaves us with the strategic management and its complexity. Honestly, I wouldn't mind some abstractions that remove tedious steps like shuffling around magazines or grenades. Before Firaxis removed it I was unsure if it was the right thing to do. I mean, extensive ammo management was part of all my beloved turn-based tactical games from the 90s, so I believed it was important. But now I wouldn't want it back and I hope JA3 will do something similar (like auto-restock ammo/grenades and auto-repair weapons/equipment in certain zones or allow resupply for a small fee via drone etc.).
Logistics should be all about acquiring special equipment for upgrading your mercs and their items piece by piece. Getting ammo or even a car is trivial, getting rare equipment like armored military vehicles, a Ratnik body armor or tactical AR goggles isn't. And that's what I wish for: an inventory system that is more than just 2 utility slots, one weapon and that's it.
Gaining experience and earning new skills/better stats is also a big part of the 'true' JA experience. I'd be okay with a few restrictions; I know that not everybody can become an expert for explosives or sniper rifles. Just not a system that locks you out of basic options.
What also grew on me is the whole 'create your own merc' thing. It was great to have at least one custom merc and I hope it is still in.
Side-quests are a must. They don't have to be huge or elaborate, but who doesn't remember Kingpin and his money?
The strategic management shouldn't be too shallow. While I think that building and upgrading a base would be nice, I don't think it is necessary. But upgrading mines, rebuilding barracks for militia or repairing an old harbor or a radar installation to unlock more options/features should be the bare minimum.
Overall I remain hopeful. The few things we were shown - including the choice of developer - are convincing. For now at least.