Amplio2 is the default tileset for the GTK client (TBD: and maybe others). Its predecessor Amplio is available as modpack for 2.4 and 2.5. Both versions support isometric (mostly 96×48) tiles suited for isometric topologies.
Variants for a hex topology exist, e.g., Amplio2hexbig modpacks for 2.4 and AmplioHexbig for 2.5. Actually you could use Amplio2 also for hex. topologies, but of course either the N-S (keypad 8+2) or W-E (keypad 4+6) connections won't work. Freeciv 2.6 includes Hexemplio based on "Amplio2hexbig".
There are lots of small improvements in Amplio2 compared with Amplio, e.g., all resources got better visible icons, cities use large instead of normal flags, units get large instead of normal shields, etc. Arguably the older Amplio ruins instead of the Battle for Wesnoth ruins are less obtrusive, as long as ruins have no effect at all on game play. Likewise the normal flags are good enough or even better than the large flags, because they take less space on the map.
At the moment you should better not simply switch the flag size in Amplio or Amplio2, because the indicator for an occupied city has a corresponding width, i.e., Amplio2 has a wider occupancy bar (used for all city styles and sizes, different tags for the same cd.occupied sprite).
Amplio had match types for mountains, hills, and forest. Amplio2 uses bigger mountains (nice) matched with mountains and (oddly) hills. Amplio2 also matches hills with hills and mountains. Amplio2 got a set of 16 jungles, after fixing the dirty transparency (= fully transparent pixels with invisible non-zero color info) 14 different jungles remain. These jungles are matched with jungles and forests, likewise the 16 forests are matched with forests and jungles. It's not obvious why forests and jungles are matched at all, for a better random effect it would be simpler to enumerate the 16 forests and 14 jungles without matching. Maybe an enumeration would be also good enough for hills and mountains, the GTK2 2.4 client can handle this. (Maybe older clients didn't support this, and the various match types for forest/hills/jungle/mountains were a workaround for a missing feature).
Amplio uses a nuke based on the old isotrident tileset nuke, Amplio2 uses a nuke based on the old trident tileset nuke. This is a matter of taste. The 3×3 nuke parts in tiles.png based on the isotrident tileset are not used and would not work for a 96×48 grid. There are still various apparently unused tags, e.g., the old cd.* tags (city defaults), the old ts.river_resources alias of ts.grassland_resources, and the t.unknown1 alias of mask.tile.
If you try to tweak Amplio2 you could stumble over a situation, where the client terminates itself, because it needs basic terrain tiles in layer 0 also for water terrains when you don't use the existing ocean.spec with ocean.png in this layer. To get a t.l0.coast1 tag add it as alias for t.l0.cellgroup_s_s_s_s (= all shallow). To get a t.l0.floor1 add it as alias for t.l0.cellgroup_d_d_d_d (= all deep). You could replace the unused dummy t.l0.cellgroup_l_l_l_l (= all land) in ocean.png by a lake for a t.l0.lake1. This is the same idea as for the fogs, there is no t.fog_k_k_k_k (= all known), and the author put a GPL info in its grid position 4:4 for fogs.
To construct a t.l0.lake1 sprite you can combine the existing W-E corners t.l0.lake_cell_l_s_s_s (left) and t.l0.lake_cell_r_s_s_s (right) horizontally, and fill the resulting 96×24 (96=48+48) bitmap with 12 transparent pixel rows at the top and the bottom for size 96×48 (48=12+24+12).
Then combine the existing N-S corners t.l0.lake_cell_u_s_s_s (up) and t.l0.lake_cell_d_s_s_s (down) vertically to get a 48×48 (48=24+24) version, and add 24 transparent pixel columns on both sides for 96×48 (96=24+48+24). Finally merge both versions, i.e., keep the one non-transparent pixel for most positions, and keep fully transparent pixels elsewhere. You can also use the uploaded t.l0.lake1 ;-)