Water combat opens up a whole new can of worms I don't think PGI would ever care to really tackle.
Otherwise...
LIGHT:
Mercury - The reason the Clans have their OmniMechs is because of the lore of this little guy. It's as light as a Commando except it's base speed is MUCH faster, though with MW:O caps it'll be just as fast as any other light 'mech. It's base variant mounts at least 4 energy hard points, and gives the devs an excuse to try to figure out how to fit MASC into the game (although I'm unsure of MASC's reintroducton into the IS.) However, most of it's variants are carbon copies so it'd have to go the way of the Spider and be given extra hardpoints.
http://www.sarna.net...Mercury.jpg.png
Panther - Why not have the Panther after putting the Dragon in game? Has enough variants to vary it up a bit, and while it might be slow (which is usually anathema to light mechs in MW:O,) it has a really strong punch for it's size while carrying jump jets. It's an alternative to the all laser boating Wolfhound, but only by so much.
Possible Hero Mechs:
Minobu Tetsuhara, friend of Jaime Wolf and founder of the Ryuken, originally piloted his family's Panther, a mech nicknamed Katana Kat. Later he would switch to a Vindicator, and later still he would pilot a Dragon. Katana Kat would pass to his brother, Fuhito Tetsuhara, who used it successfully during the Fourth Succession War.
Tomoe Sakade, wife of Theodore Kurita, used a Panther during her early career, up until its destruction during the Fourth Succession War.
http://www.sarna.net...25_Panther1.jpg
Wolfhound - The Wolfhound is definitely a favorite in universe and out, and I'd personally love to see this little 'mech make it into MW:O. It's fast enough to be as fast as the rest of the lights while still mounting great armor and weaponry, and Alex's concept art would probably be mind blowing (like all the others are.) I've been a fan of this 'mech for a VERY long time, and it's my favorite for next light to be included...but I'd be happy that it got in at all down the road. Also...
Possible Hero Mechs:
Daniel Allard, who received one of the prototypes in 3028 after the destruction of his Valkyrie in 3027. Originally a stock WLF-1 model, it was later upgraded to WLF-2 standard and finally with Clan-tech weapons salvaged from the Battle of Luthien. Allard continued to pilot his Wolfhound even after rising to overall command of the Kell Hounds up until his death on Graceland in 3069 during combat against Clan Jade Falcon.
Phelan Kell, son of Morgan Kell, also piloted a Wolfhound while serving with the Hounds until his capture by Clan Wolf in 3049. After being dissected and studied by Clan scientists, when Phelan was granted full warrior status Khan Ulric Kerensky authorized the reconstruction of his Wolfhound to become the aforementioned Wolfhound IIC variant.
http://www.sarna.net...f/Wolfhound.jpg
MEDIUM:
Clint - Fast, jumps, and has a lot of variants. I like it much better then the alternatives (unless we get the Unseen/Reseen, covered later.) Another 'mech I'd like to see Alex tackle, visually.
http://www.sarna.net...3025_Clint1.jpg
HEAVY:
Axman - Forget a second that it's got a melee weapon, which pretty much nulifies this mech just by that virtue. Or don't, and just read on anyway for the fun of it! It's kinda slow (but that's fixable,) and it carries jump jets. Has a decent amount of variants (one with ECM!!!) and pretty much any Mechwarrior knows this Battlemech, in universe and out. Could always go the route pictured below to fit it into the game.
Possible Hero Mechs:
Adam Steiner piloted a AXM-2N variant at the opening of the Clan Invasion. The 6X variant was developed primarily in the hopes of pandering to the Archon's use of the Axman during the Clan Invasion.
Information is ammunition, baby!
http://www.sarna.net..._Axeman.jpg.png
Guillotine - I'd prefer this over the other non unseen/reseen Battlemechs. Old Star League design with decent variants (though, not super varied,) mounts jump jets, and would just look cool redone.
http://www.sarna.net...try_%282%29.jpg
ASSAULT:
Hatamoto-Chi - Because Space Samurai. I'm drawn to this 'mech aesthetically. While I really want this or the Cyclops to be the next Assault, I'm doing it purely off of how it looks because it's variants don't vary at all. It's a cooler looking Thug, and has a lore link to the Charger that's kind of interesting.
Possible Hero Mechs:
HTM-27T Hatamoto-Chi Daniel - Personally awarded by Coordinator Takashi Kurita to Sorenson’s Sabres commander Daniel Sorenson in the lead-up to the War of 3039, the 'Mech Daniel received was a bridging design between the early field-test HTM-26T and the later production HTM-27T. Dropping two tons of armor and outfitted with a bulkier and less refined Endo-Steel chassis, enough weight was saved to mount a pair of experimental Luthien Armor Works developed LB 10-X AC prototypes in it arms and a pair of Telos SRM-4 launchers in its chest. A ton of reloads for each SRM launcher and four tons of autocannon reloads in its CASE protected side torsos kept the 'Mech in the thick of battle.
http://www.sarna.net...amoto-Kaeru.jpg
Cyclops - This 'mech needs a redesign visually from Alex, and it'd be a good solid choice for the next Assault due to it's weight and decent variants (though, many are almost identicle but that can be padded out.) Good candidate for ECM on another Assault, and is often seen as another strong command style mech. This is the 'mech I want to see above all others as the next Assault.
Possible Hero Mech:
General Ariana Winston, Commanding Officer of the Eridani Light Horse between 3045 to 3060, piloted a modified Cyclops which featured a dual-cockpit for command and coordination purposes. She is perhaps best known for assuming command of the reborn SLDF's Task Force Serpent after Morgan Hasek-Davion's assassination and leading the bloody campaign to destroy Clan Smoke Jaguar on their homeworld of Huntress, though she was killed during the effort.
http://www.sarna.net...yclops1.jpg.png
RESEEN/UNSEEN: (Posting while fully knowing we may NEVER seen the Unseen.)
Valkyrie - Small, fast, can jump, and has a number of variants. Can't do much with 30 tons and it really ends up being another Spider, but we need some new takes on these old Unseen so we can get over all the hooplah once and for all. I've always liked the Valkyrie, and in the end the light field is covered with anything unique already with what's in game...it's mostly preference at this point and I'd prefer to see this.
http://www.sarna.net...5_Valkyrie1.jpg
Ostscout - It's a pod with legs, but hoping Alex would change that (as he has done so awesome for so many 'mechs.) I've liked this 'mech in table top and prefer it over Ravens for various reason, not all due to their perfomances. This 'mech really wouldn't survive MW:O due to it's paltry and bad variants as electronic warfare is important...but not nearly as powerful as it is in TT.
http://www.sarna.net...stscout.jpg.png
Phoenix_Hawk - This damn machine has more variants then you can shake a stick at. Lower end of the weight scale for Mediums, but it's got it all where it counts. It's a real workhorse medium and a 'mech I'd dearly love see get a redesign. Especially because of...
Possible Hero Mech:
Shin Yodama piloted a Phoenix Hawk for much of his early career in the DCMS, including the Clan Invasion.
http://www.sarna.net...x-3025phawk.jpg
Warhammer - I don't even have to write a description. I really don't. But I'm going to anyway! Probably the most iconic Battlemech in all of Battletech, the Warhammer is the machine most people first think of (unless they're dirty Clanners,) and is basically the face of Battletech for most of us. It has a good spread of weapons, it sits at 75 tons making it a good weight to allow for customization...and well it's the bloody Warhammer! It's not like one of the two most deadliest pilots in all of the Inner Sphere pilots it or anything and is the most high profile, even above Kai Allard-Liao (who's technically in game already.)
Possible Hero Mechs:
*Natasha Kerensky piloted a jet black Warhammer while still serving with Wolf's Dragoons. (Who doesn't love the Black Widow?)
Colonel Donovan Fresnel of the 75th Royal Hussar Regiment became the first SLDF "Gunslinger" to achieve a draw against a DCMS Marauder in 2687 on the world of Minakuchi during an hour-long duel.
Tai-sa Yorinaga Kurita was a House Kurita warrior who gained notoriety for killing Ian Davion and battling mercenary Colonel Morgan Kell.
http://www.sarna.net...rhammer.jpg.png