This is somewhat of a dilemma for me. On one hand I would enjoy the variety in one army and the extra viability of currently underused units, but on the other hand with the way GW has taken these factions I would like to see what they can do with each theme when isolating it and focusing on expanding it.
For example, Firebellies on their own are a completely worthless faction, but what if they were fleshed out with a cool pacific islander theme? Aleguzzlers are a useless faction, but what if they were made to be an army entirely of giants, similar to Imperial Knights in 40k? Spiderfang and Troggoths can be fleshed out easily as well, both have strong themes. Greenskinz are quite a unique GW specific design, but I would be fine if Gitmob were rolled into them, as well as Maneaters being rolled into Gutbusters. Skaven are more of a candidate for a merged force in my eyes but they can still do a lot with each clan on their own.