I think that whilst you may be fairly restricted in weapon-swaps on the carapace weapons, you have fairly free reign over magnetising the arm weapons.
Many 40k weapons look pretty good on Epic titans as long as you consider some of the scale-detail and either hide or disguise it. Also consider that the 40k weapons do need to be fitted with a proper 'arm' or they will also look a bit odd.
Following are a few weapon ideas (some of which Ragnarok has already done), and I hope you find some of the suggestions helpful:
Gatling BlasterAssault cannon. I suggest looking at the plastic Terminator one as it is a bit smaller than those on the Ladnspeeders. Otherwise, the left arm gun on the standard Mk3 Titan works perfectly well!
Inferno Gun40k flamer muzzles and plastic tubing. I would suggest 2-3 muzzles side-by side and then plastic barrels to suit.
Laser BlasterQuad barrelled weapon using 40k Chimera lasguns would work well. Or normal lasgun/laspistol muzzles with plastic tube barrels.
Turbo LaserOld plastic Laser Burner with the barrels replaced with a pair of the current Warlord carapace weapons. See Carl Woodrow's titan below for an example.
Corvus Assault Pod40k plastic Landspeeder Typhoon Missile pod with a plasticard door (and you may want to trim the missiles off too!)
Multiple Rocket LauncherHalf of the 40k plastic Terminator Cyclone Missile unit, and the new Landspeeder Storm's missile pod is quite good for carapace mounts too.
Quake CannonI'd dress up a plasticard tube with suitable gubbins, but any large-bore gun would do (40k IG Autocannon with a cut-down barrel and the ammo hopper removed?).
Volcano CannonThe right arm gun on the standard Mk3 Titan works perfectly well! Otherwise, I suggest using a 40k Lascannon with a new muzzle and the power cables trimmed off and replaced with some that are much smaller. You could also try to tart up the old Mk1 plastic weapon with a new muzzle and some extra detailing.
Inferno Gun40k meltaguns or a multimelta would work. I'd use the Landspeeder's MM or perhaps the the Immolator's though as they aren't as 'tall' as the infantry-portable ones. The Demolisher has some MM sponsons that could be bulked up as well.
Plasma Blastgun40k plasma gun bulked up with bits. Or maybe a pair of plasma pistols? Needs to look lighter than the other plasma weapons.
Plasma Cannon / Plasma DestructorTwo 40k plasma guns or a 40k plasma cannon. Maybe consider the new the Demolisher has some plasma cannon sponsons that could be bulked up.
Vulcan Mega Bolter40k heavy bolter or bulked-up stormbolter. Or a pair of 40k Rhino/Bike bolters.
Carapace Landing PadAnything flat with a little lip around it. I've seen slottabases with a brass aquila on them look very good! I consider the spotter craft to be optional.
Fire Control TowerAnything boxy like SM missile pods or a cut-down 40k 2nd Ed Hunter Killer Missile box with plenty of antennae and radar dishes on it (BFG plastic cruisers are a good source of these bits).
Power FistSM power fists look good (need to construct the arms though!), and so do the Chaos SM ones (remove the symbols etc, but leave the taloned fingers). 40k Servo arms are also a decent source of grabby-bits, as is some of the Inquisitor scale stuff. Also check out some of the Khadoran Warjacks from Warmachine (you can get fists as parts on Ebay, or from their online parts store).
Chain Fist40k eviscerators, IG Catachan Sentinel saws, Inquisitor scale chain weapons are all good. Even the old plastic Mk1 Warlord saws look good if you keep the blade and replace the upper arm, and the Mk2 metal warlord saws look good as-is.
Support MissileTrim a 40k Hunter-killer missile to suit. IG or SM missiles look different, but both are equally good IMO.
Carapace Multi LasersBFG plastic or metal cruiser turrets are brilliant. Epic Ork battlewagon turrets with the guns replaced by lengths of thin rod look good too.
Laser BurnersLascannon with the barrel removed; just glue the muzzle straight onto the weapon's base. I've added an example to this post, but I'd probably replace the power cables to something more in scale as well.
