If you want more lag, make planetary clusters orbit suns/rogue stars, moons/belts/anomalies/etc... orbit planets, and planetary structures that orbit planets/moons. The game will constantly be updating phase lanes or jump coordinates (assuming a free movement system comes to SINS).
Take it from CCP -designers of EVE ONLINE- Newtonian Physics are overrated. But I would like to see ships moving DURING combat -just a little so I don't get sleepy; yet not so much that people vomit while playing intoxicated... well, actually that might be funny as hell. Buddy got hate-mailed for ninja tackling a Megathron in his uberized Ishtar (7900m/s) and making the guy puke by orbitting so fast for 3 minutes -he was drunk and ratting in 0.0 (bad combo)
Z-axis strategies are great. Move your fleet up/down to evade turrets that are weak as hell, fighters that can be crippled with cap abilities/anti-fighter frigs, and run the other player all over hell and breakfast only to jump out while another force jumps in to safely kill structures. However, reintroducing Z-axis means reworking planetary defenses so Z-axis use doesn't give such overwhelming superiority -it imho should stay cosmetic and for repositioning ships. Turrets would need much longer range and firepower to combat a fleet intentionally outside it's range spamming bombers to neutralize them, Fighters/bombers would need a greater speed buff, more firepower, and less affected by anti-fighter frigs/cap abilities to fend off a dozen cruisers with bombers and a compliment of anti-fighter frigs.
I wouldn't mind a narrow Z-axis of freedom, especially after designing maps where every planet(oid) is on the flat plane of the sun to which it belongs -again, even random Z-axis positioning within a narrow band would be nice (+/- 300 on the Z-axis of the parent sun). Flat maps suck. I love the Rogue Stars -lets me make a map that doesn't feel fake.
