A base seems very unlikely to me. One that doesn't transform or come with several exclusive transforming figures is frankly right off the table. There's no way IMO they're going to do a Haslab Anniversary project that doesn't have the brands main gimmick and reason for being front and center.
Possibilities I could see:
*The Nemesis or Primus as counterparts to existing Titans. These don't feel super-likely because they are both less iconic than their counterparts that have already been made, but they're big enough bough and prominent enough to be possible. Especially if Hasbro put some major UMPH into their design.
* Really Really Masterpiece Masterpiece Prime. This is unlikely because it'd break their own rule AND because what more could they even do that hasn't been done before. But it's the super low hanging fruit of an Anniversary thing.
* Really Really Good G2 Tank Megatron - If you can't/won't do Optimus he's next in line for obvious anniversary go to. And if you can't do the Gun this is the next Iconic option, and there's not a really really great on model tank Megatron toy. Pros: Lots of potential. Cons: Second choice on several fronts.
* Some sort of G1 gift set. I don't know what but I'm stretching for things that would justify a HasLab. Too many 84 bots for a gift set. Dinobots are out. Devastator is the obvious choice but doesn't excite me loads. I'm decently satisfied with the Titan and I'm not convinced they'd pull it off better even at Haslab level.
-ZacWilliam, things that would excite me personally: Big Red, Spanner, Scrounge and Smelting Pool, fully featured Pretenders, Prima and Magatronus, New Cool Characters are all WAY too niche for them do a Haslab, especially for the anniversary.