So here's a little walkthrough of the Orks and 'non-radical' shipbuilding (I.E. no new/weird additions to current rulesets, ships follow general theme of orks)
Weapons:
Guns, usually D6's in any circumstance above firepower two. Presuming we're staying on a standard 'evens' system this would produce 4 firepower. Additions of +2 are common as this would raise it to a 'fp6'. 2d6 not present, instead we have D6+4 to represent fp'8' and D6+6 to represent fp'10'. Guns are only ever longer ranged in the forward location, forcing players to face their enemy.
Heavy Guns, Common either firepower 4 or 6.
Torpedos: Either listed as D6 or slightly increased in strength as a D6+2.
Bombardment cannon/lances: Character only (thus I unallowed in this scenario, and unfluffy for orks save on the largest of ships)
Launch Bays: Either 2 per side or D3+1 dorsal.
Soopa Engines (listed here as they represent a hardpoint)
Weapons Always fire unidirectional, unless on a defense. Firepower will always be greater in the forward arc, as this fulfills the theme of orks.
Chassis:
Cruiser: 10 hits 1 shield, 1 turret, forward armor bonus compensated with negative rear armor
Above average hits, half shields and turrets compared to imp/chaos. Speed lower as well
Battleship: 12 hits, 2 shields, turrets 3?
Standard hits, Half average shields, near average turrets.
Noting that everything in an ork arsenal is lower quality than that of imperials or chaos in every way save for pure damage taking capacity. Other trends note that no vessel (other than escorts) will have a better turn radius than 45. Presumably an ork CL would be two less hits than that of a Krooza, but it is possible (as shown by their battleships) that they could have the same value. Presumably Ork ships are around 15% cheaper than ships of similar class from other fleets. This would mean that an ork CL should cost about 100 pts.
General trends in BFG show a system of hardpoints; in orks this is F:2,S:2,D:0 for cruisers, F:2,S:2,D:1 for battlecruisers, F:2,S:3,D1 on Battleships. Presumably from what we know about imperial light cruisers (the closest thing to potential ork variants) they have F:2,S:1,D:0
Now with these limitations we will set four threshold values, Minimal, Light, Medium and High. CLs take minimal weaponry for sides, Light for front. Cruisers take light for sides and Medium as front, Battlecruisers take medium for sides, and high for front. Battleships take medium for sides and high for front.
For guns we note: Mi:2 L:d6 M:D6+2 H:D6+6
Heavy Guns: L:4 H:6
Torps: L:D6 H:D6+2
Launch Bays: L:1 M:2 H:D3+1
Now assuming that BBs are already covered, and BCs are a lot less needed without list changes our three categories needed for larger development are: Cruiser (primary goal), Light Cruiser, and escort. Now the thing is to solve problems in the ork fleet without reducing viability of current designs (not stomping on a kill-krooza and making it useless by making a better, cheaper, faster version, looking more towards something different), and without breaking current established themes and weaponry options. Basically we have to bash two rocks together and somehow make something miraculous yet simple that falls within current orky theme.
anyways, for now I'll list off a few ideas in each category:
Cruisers:
Assault cruiser: This ship would be of course based off the kill-krooza stats, but would replace its heavy guns or side guns for soopa engines. To not make it cross with the kill krooza and keeping it unique I would lose it's long range weapons batteries, instead replacing them with torps and keeping the heavy guns. I saw a previous poster mention a 'ram' and I could see this as adding +4 to the ships 'hits' when it makes ramming attemps, but would likely count as a hardpoint. Possibly you could have mega armored orks free on the vessel, as well as boarding torps.
Torpedo Carrier, basically a vessel that has replaced it's heavy guns on all sides in favor of torpedos, (it would give a good opportunity to buy the looted torps upgrade) These torpedos would only fire in each of their respective directions.
Gunboat: Likely anything of this type would shroud the Kill-Krooza, so it's difficult to make designs for it, but certain ideas are possible.
Light Cruisers:
Naturally light cruisers would need to have similar disadvantages to impie and/or tau CL's where they are slightly too squishy to be used in replacement of the standard, rather better in conjunction with them. I which would warrant either 6/4 armor (similar to escorts) or only 6 hits or even possibly both. Or the other option is to keep their cost relatively high and increasing their side armament (whilst keeping them in the light/minimal variety) to two as shown by my previous example. This would effectively make them a slightly lighter Krooza (which feels a bit more orky than the agile ships of the imperium) and still deter people from buying 4 CLs to get to the restricted BB/BCs.
Assault Ship: Similar to that above, likely a larger verson of a savage.
Gunboat/Mixed: Like the Kill-Krooza
Carrier: A smallish Carrier
Escorts:
Escort Carrier: Yes... I'm putting this up there